.travis.yml: Return apt-get update

Since we have to install extra packages in the VM, some
of which might have been moved/updated, apt-get update
turns out necessary.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #278 from lmr/fix-travis-apt-get

.travis.yml: Return apt-get update