.travis.yml: migrate to container builds

This moves the Travis tests from the legacy VM infrastructure (which
only seems to run 5-6 jobs at once) to the new container based approach.

The principle difference is there is no sudo in the containers so all
packages are installed using the apt add-on. This means one of the build
combinations can be dropped as it was only for checking the build with
additional packages.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>