Travis: route builds to their new infrastructure and enable caching

[dequis]

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 from 1.1.2 to 1.2.0. See their changelog:

Convert ShadowJar.groovy to ShadowJar.java to workaround binary incompatibility introduced by Gradle 2.2

~/.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)

[turt2live]

This looks interesting and quite useful, however it appears to be better suited as a discussion/private message on IRC instead.

Thanks for your work, however! I hope to see this (or similar) soon!