Why is installing the json 1.8.6 gem on Windows 8 not working?

I've cloned down a Rails app and am trying to install all the gems on my PC. I'm running Windows 8.1 x64, and I've installed the x64 devkit at C:/devkit and am using uru to manage ruby versions so that I can use ruby 2.3.1.
When I run bundle install or gem install json -v '1.8.6' -r --source http://rubygems.org/, I get the following error log:
    current directory: C:/Ruby23/lib/ruby/gems/2.3.0/gems/json-1.8.6/ext/json/ext/generator
C:/Ruby23/bin/ruby.exe -r ./siteconf20170216-17824-1b67o5.rb extconf.rb 
creating Makefile

current directory: C:/Ruby23/lib/ruby/gems/2.3.0/gems/json-1.8.6/ext/json/ext/generator
make "DESTDIR=" clean

current directory: C:/Ruby23/lib/ruby/gems/2.3.0/gems/json-1.8.6/ext/json/ext/generator
make "DESTDIR="
generating generator-i386-mingw32.def
compiling generator.c
In file included from c:/Ruby23/include/ruby-2.3.0/ruby.h:33:0,
                 from ../fbuffer/fbuffer.h:5,
                 from generator.c:1:
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:131:14: error: size of array 'ruby_check_sizeof_voidp' is negative
In file included from c:/Ruby23/include/ruby-2.3.0/ruby.h:33:0,
                 from ../fbuffer/fbuffer.h:5,
                 from generator.c:1:
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_obj_freeze_inline':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1360:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1361:2: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1362:6: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1362:27: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_data_object_get':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1400:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_data_object_make':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1414:5: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_data_typed_object_make':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1421:5: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_num2char_inline':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1587:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1587:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1587:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1587:41: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1587:41: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1587:41: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1588:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1588:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1588:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_class_of':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1949:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_type':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1966:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
In file included from c:/Ruby23/include/ruby-2.3.0/ruby.h:33:0,
                 from ../fbuffer/fbuffer.h:5,
                 from generator.c:1:
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_clone_setup':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1997:56: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1998:33: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1999:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:1999:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_dup_setup':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2005:42: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2006:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2006:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_array_len':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2012:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2013:2: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2013:24: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_array_const_ptr':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2027:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2027:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2027:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_struct_len':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2034:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2035:2: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2035:26: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h: In function 'rb_struct_const_ptr':
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2041:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2041:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c:/Ruby23/include/ruby-2.3.0/ruby/ruby.h:2041:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
In file included from generator.c:1:0:
../fbuffer/fbuffer.h: In function 'fbuffer_append_str':
../fbuffer/fbuffer.h:127:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
../fbuffer/fbuffer.h:127:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
../fbuffer/fbuffer.h:127:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'convert_UTF8_to_JSON_ASCII':
generator.c:135:35: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:135:35: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:135:35: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:136:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:136:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:136:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'convert_UTF8_to_JSON':
generator.c:233:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:233:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:233:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:234:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:234:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:234:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_s_allocate':
generator.c:551:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_configure':
generator.c:572:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:572:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:572:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:573:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:573:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:573:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:580:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:580:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:580:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:581:33: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:581:33: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:581:33: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:588:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:588:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:588:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:589:40: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:589:40: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:589:40: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:596:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:596:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:596:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:597:36: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:597:36: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:597:36: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:604:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:604:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:604:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:605:37: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:605:37: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:605:37: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'set_state_ivars':
generator.c:655:24: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:655:24: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:655:24: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'generate_json':
generator.c:892:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:892:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:892:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'isArrayOrObject':
generator.c:953:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:953:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:953:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:954:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:954:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:954:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_indent_set':
generator.c:1078:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1078:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1078:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1087:32: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1087:32: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1087:32: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_space_set':
generator.c:1116:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1116:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1116:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1125:31: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1125:31: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1125:31: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_space_before_set':
generator.c:1152:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1152:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1152:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1161:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1161:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1161:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_object_nl_set':
generator.c:1190:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1190:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1190:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1198:35: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1198:35: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1198:35: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c: In function 'cState_array_nl_set':
generator.c:1225:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1225:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1225:11: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1233:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1233:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
generator.c:1233:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
make: *** [generator.o] Error 1

make failed, exit code 2
I've tried:
installing json_pure and specifying it in my gemfile for use with :mswin & :mingw
running bundle update
removing everything with spaces from my PATH variable
going through the list of devkit troubleshooting options here
I've also trawled through a good number of other SE questions that are similar, but the one that looks most relevant to my situation remains unanswered.
Any more suggestions about what could be going wrong?
Solution
You have mismatched platforms.
generating generator-i386-mingw32.def is the x86 version of Ruby and you said you are using the x64 DevKit.