Configure Travis CI branches.

This commit updates the configuration for Travis CI to build the release
branches, as well as the 'develop' branch. This should make it so that pull
requests on those branches automatically get a notification about whether the
code builds correctly.
Merge pull request #421 from flerda/release-2.2

Configure Travis CI branches.
Merged '45beecc' from develop: Configure Travis CI branches. This commit updates the configuration for Travis CI to build the release branches, as well as the 'develop' branch. This should make it so that pull requests on those branches automatically get a notification about whether the code builds correctly.