Add travis config for CI

Travis will automatically run the specs for various versions of ruby.

I enabled `1.9.3`, `2.0.0`, and `2.1.2` (which is they're latest non-custom one). I wasn't sure if you intended to support 1.8.7 so I left it out.

To get this working, follow [steps 1 & 2 of the getting started](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in), and then merge this PR.