tests: py.test instead of nose

* Replaces nose by py.test because it has colours (and is maintained).

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
travis: gitter.im integration

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
herp derp: breaking the build.

Signed-off-by: John Doe <john.doe@example.org>
global: pytest rewrite

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>