Add .travis.yml

:convenience_store: 

This is the first time I've enabled travis for a cookbook, so I'm definitely open to suggestions if the file I made doesn't look quite right.

Closes https://github.com/opscode/supermarket/issues/887
`kitchen test` does `kitchen setup && kitchen converge && kitchen verify && kitchen destroy` if I'm not mistaken, so I don't know if we need this.