Migrate from legacy to container-based infrastructure (Travis CI)

[EivindArvesen]

Migrate infrastructure on Travis.
There are some useful things to be found commented out in .travis.yml.

Needs to be tested, ensuring that testing actually is faster on the new infrastructure; i.e. make sure that the substituted before_install steps aren't actually slower.

Install BATS from source, or wait until its APT source is whitelisted by Travis CI.