Setup Travis CI

You'll need to set the repo up in Travis (it's three clicks and I can help if you need it) and change the URL [here](https://github.com/dball/data_uri/pull/2/files#diff-cb3e0f2c76a671c083e8f001970f4631R1).

You can see it running the tests for my fork [here](https://travis-ci.org/porras/data_uri).