Add a .travis.yml

#85 mentioned the desire to get travis-ci set up, so I did it in a basic way.

Currently runs two builds on ubuntu test runners, on python 2.6 and 2.7. 

Travis is showing two failing tests on master, but as far as I can tell, that's not a misconfiguration on my part but rather that the tests are actually failing. (I'm getting the same result privately.)