travis: Add a Travis CI settings file

Although I'd rather have a continuous integration setup on our own (like next to git.geany.org), I'm afraid we don't really have time to set it up nor probably resources to run it on almost everything.

So, maybe [Travis CI](//travis-ci.org) is not so bad -- though although it doesn't say so anywhere, I don't know if it comes with strings attached.  Anyway, here is a *.travis.yml* that [seems to work great](//travis-ci.org/b4n/geany).  If we agree on it, we can merge and then enable the repository in Travis CI.