Travis: route builds to their new infrastructure and enable caching

Their new container based infrastructure starts builds faster, takes
half of the time to finish, and allows us to do caching of dependencies.

One random (but very nonscientific) test:

Old: 1m54s in queue, 2m13s running, 4m07s total
New: 0m14s in queue, 1m02s running, 1m16s total

The main downside is that that root access it not available, but it was
only used for installing jq which was also added to the build
environment recently.

These new servers also include gradle 2.2.1 instead of 2.0, which
required an upgrade of the shadow plugin.

~/.gradle and ~/.m2 are cached, and a few dirs in .gradle are removed
at the end of the script to avoid unnecessary cache updates

Also the "clean" step was removed to save a few seconds of build time,
since most of the time it doesn't download libraries (not anymore)