Q: Rails install suddenly requires concurrent-ruby gem and fails

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?