travis: re-enable plainbox testing

Travis is a valuable and free service. We should try to test plainbox
code with travis even though we don't use git as our primary code
hosting service.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
travis: update travis image marker to track checkbox

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
Merge pull request #1 from zyga/launchpad/travis

Launchpad/travis