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
I think the errors showing on https://travis-ci.org/opscode-cookbooks/supermarket/builds/39204296 are telling us that we need to `berks install` first, probably also want to try setting `BERKSHELF_PATH` to somewhere where we can cache it.