adding a .travis.yml file for travis-ci.org

I've added a ``.travis.yml`` file so that the test suite can be run automatically by travis-ci.org. All you need to do is sign in to https://travis-ci.org with your github account and turn on this repository. Then the tests will be run on every push and pull request. You can see what a test run looks like here: https://travis-ci.org/mhrivnak/keepassx