Add Travis CI

I cloned this repo and noticed the tests were failing on the `master` branch. The best way to be notified about test failures is to setup continuous integration, so I’ve done that with Travis CI.

This patch also adds a badge to the `README` to make the build status more visible.

All that is required after merging this patch is enabling Travis CI for this project.

Everything is done in a separate commits, so feel free to cherry-pick only the changes you want.

Thanks!