Update Chef versions & Ruby versions for Travis

Run the last patch release for each previous version of Chef 11, and
only run for the Ruby version that is packaged with each version, i.e.
Ruby 1.9.3.
Add (experimental) support for systemd as an `init_style`

First stab at getting `systemd` to work. Still not sure about a few
things such as using an EnvironmentFile and the code for setting up
everything related to init styles is even messier, so it could probably
use some cleanup and rethinking.

But hey, it works.