Glib TLS support is required to run some of the tests

Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>

Travis host images are quite outdated, so the builds will now use containers
instead. By default there's a build for Arch Linux (using base/archlinux:latest
image) and Fedora (using official fedora:latest).

Builds are done using extra/travis-build script.