Fixes #129 (Github Issue).

Unfortunately, we rely on `service_facts`, which is only available
in Ansible >= 2.5.

There's little reason to stick to an old version such as Ansible 2.4:
- some time has passed since we've raised version requirements - it's
time to move into the future (a little bit)
- we've recently (in 82b4640072) improved the way one can run
Ansible in a Docker container

From now on, Ansible >= 2.5 is required.