Update .travis.yml
Update .travis.yml

A new version of Ruby has come out since we last updated this, so I
added it to our testing matrix. I also changed 2.1.0 to 2.1 so that
2.1.x will be tested instead of specifically 2.1.0.

Specifying language will help Travis do the right things for this
project. Setting `sudo: false` will allow Travis to use the faster
container-based infrastructure [0]. And `cache: bundler` will enable
Travis to persist the bundler directory between builds, making our
builds faster [1].

[0]: http://docs.travis-ci.com/user/workers/container-based-infrastructure/
[1]: http://docs.travis-ci.com/user/caching/#Caching-directories-(Bundler%2C-dependencies)

diffux/diffux-core,diffux/diffux-core