For more information see http://concurrent-ruby.
- https://devcenter.heroku.com/articles/rack
- https://devcenter.heroku.com/articles/ruby-versions
- http://bundler.io/v1.3/gemfile_ruby.html
- https://blog.heroku.com/archives/2012/12/13/run_jruby_on_heroku_right_now
- http://rubini.us/2013/02/26/heroku-cedar-rubinius-2-puma-2-in-three-easy-steps/
- rubinius/rubinius#3210