Travis: fix build config validation warnings

Last year, TravisCI announced the Build Config validation [1][2]
beta feature. I recently noticed it was now enabled on our repo, and
yields some warnings, so this commit addresses these.

Make sure we don't rely on default values, so our tests do not fail
unexpectedly should Travis change the settings. Set the following,
matching Travis's current defaults:

• os: linux
• dist: xenial

Also, rename matrix to jobs (alias), and add the new name key for
the Documentation build job.