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
After this change, Travis will run the tests twice with the **same** Ruby version, since today `ruby-head` equals `2.2`.

Rails core team, which option do you think is better?

1) Keep both lines, so we are ready for the day in which `ruby-head` and `2.2` won't match anymore?
2) Keep only one line, so we use less resources on Travis CI, and we will re-edit this file once a new `ruby-head` comes out that differs from `2.2`.

Personally, I would go with 2) because I don't expect `ruby-head` to be different from `2.2` in the next months… but I'd like to know your opinion! Thanks :christmas_tree: