Ruby 2.1.5

Bump us up to Ruby 2.1.5.

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/

https://devcenter.heroku.com/changelog-items/552
Bump Travis to 2.1.5 too
Merge pull request #120 from heroku/brandur-215

Ruby 2.1.5