travis: speed up

• Redesigns .travis.yml to use the new container-based infrastructure. This lowers the waiting time for the queue, gives us the ability to use caches for apt and pip and improves testing time down to ~15min. We are not allowed to use sudo any more but we do not need it anyway, because Apache is not tested.

Signed-off-by: Marco Neumann <marco@crepererum.net>