Lock rspec versions in `gemfiles/Gemfile*`

Travis keeps installing 2.99 which apparently doesn’t work.
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.