Add travis build configuration.

To enable this, you'll need to go to https://travis-ci.org/, sign up and authorize github, and then tell travis to run on the hypothesis project: http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in has good instructions.

You might want to tweak the set of python versions supported: I enabled this on my fork, and get some interesting errors: it works on python 2.7 and on pypy, though! https://travis-ci.org/rboulton/hypothesis/builds/5747652