Rails install suddenly requires concurrent-ruby gem and fails

Question

After a system upgrade to macOS 10.14 I am suddenly unable to install Rails 3.2.5 on Ruby 1.8.7, since a new gem is required now: Concurrent-ruby.

I was able to install and use this system on macOS 10.13. Why is concurrent-ruby suddenly a requirement? How can I find a way to get on without it?

Answer 1

I found a solution.

I used to install some gems before installing Rails 3. These were:

rdoc-data -v 3.12
i18n -v 0.6.11
execjs -v 1.4.0 
rack-cache -v 1.2
rails -v 3.2.5

I found out that I had to install

rack -v 1.4.7

before installing

rack-cache -v 1.2

Now I am again able to install Rails 3.

Somehow the install process must have loaded the requirements for a newer version of i18n, since later versions of this gem require concurrent-ruby.