pip installing Cython should work fine as well. We'd just have to install pip first (because I had to deactivate the automatic travis-ci virtualenv) or do everything in a new virtualenv.
If you know better ways to write this `.travis.yml` file (maybe adding sytem site-packages to PYTHONPATH or whatever) feel free to make a PR against this branch or master.
(I thought it would take me 15 minutes to write this file, but it turned out to be quite hard to get this working).