travis: Use docker-based builds, new baseline

travis: Use docker-based builds, new baseline

Perform Linux builds in docker images, which allows us to freely
define the build environment rather than relying on the Travis host
system.

Use Ubuntu 16.04 "Xenial" as the new baseline for Quassel, allowing
us to make use of a more modern toolchain.

Reorganize and modernize travis.yml in general.