Add .travis.yml

As a part of http://24pullrequests.com, I've enabled http://travis-ci.org support for your repo.
You can see a passing build here:  https://travis-ci.org/joshmoore/bup/builds/15925322

If you merge this PR, you'll need to activate the bup project on travis-ci.
See http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in for more information.

All the best,
~Josh