Set up with Travis CI

To encourage keeping the build clean and to prevent regressions, automatically run the test suite on [Travis CI](https://travis-ci.org).

Note that the badge image is currently 404ing, since I set up Travis CI on my own fork of this project but put in a badge against the main repo. You can set it up for free by following the first couple of steps on [the Travis CI getting-started guide](http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in).

Links
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* Build status badge on my fork: [![Build Status](https://travis-ci.org/pbyrne/coffeeshop.png)](https://travis-ci.org/pbyrne/coffeeshop)
* [Travis CI running the builds on my fork](https://travis-ci.org/pbyrne/coffeeshop)