Require Ruby 2.2 for Rails 5.0

Stems from [this comment](https://github.com/rails/rails/pull/18203#issuecomment-68138096) by @robin850
and by the blog post http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final
@robin850 Is this the kind of change to `railties/lib/rails/ruby_version_check.rb` you were talking about in your comment, right?