Bundle doesn't want to install a gem (not yet checked out)

I want to run a Ruby on rails server, but a certain gem doesn't want to install. This rails program is not compatible with ruby 2.0, so I want to use ruby 1.9.3, which I installed, but I can't bundle install anymore...

$ bundle install
# ...
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
$ bundle exec rails s
git://github.com/goodtouch/jbuilder.git (at master) is not yet checked out. Run `bundle install` first.
$ bundle show jbuilder
/Users/julien/.bundler/ruby/2.0.0/jbuilder-7fb0eae924c7
$ ruby --version
ruby 1.9.3p545 (2014-02-24 revision 45159) [x86_64-darwin13.1.0]

In the Gemfile, I specified ruby "1.9.3", then I ran bundle install again and got:

Your Ruby version is 2.0.0, but your Gemfile specified 1.9.3

The Gemfile line for this specific gem:

gem 'jbuilder', git: "git://github.com/goodtouch/jbuilder.git"

And what I get in the Gemfile.lock:

GIT
  remote: git://github.com/goodtouch/jbuilder.git
  revision: 7fb0eae924c77318ede344f32a5c208b91cdbaac
  specs:
    jbuilder (0.4.1)
      activesupport (>= 3.0.0)
      blankslate (>= 2.1.2.4)

I really don't understand what's going on. I'm under Mac OS X.

Thank you.

Solution

Edit: You have to use rvm or rbenv to set ruby version. This is not set in the gem file. Ruby is not a gem.

This error might have one of many reasons.

From the Bundler issues

# remove user-specific gems and git repos
rm -rf ~/.bundle/ ~/.gem/bundler/ ~/.gems/cache/bundler/

# remove system-wide git repos and git checkouts
rm -rf $GEM_HOME/bundler/ $GEM_HOME/cache/bundler/

# remove project-specific settings
rm -rf .bundle/

# remove project-specific cached gems and repos
rm -rf vendor/cache/

# remove the saved resolve of the Gemfile
rm -rf Gemfile.lock

# uninstall the rubygems-bundler and open_gem gems
rvm gemset use global # if using rvm
gem uninstall rubygems-bundler open_gem

# try to install one more time
bundle install