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
Looks like 2.2 isn't officially available yet in Travis: http://twitter.com/travisci/status/548473150833709057

Lets remove 2.1 and 2.2 for now and keep ruby-head (since it's 2.2 right now)