Add Ansible guide and Ansible version checks

After having multiple people report issues with retrieving
SSL certificates, we've finally discovered the culprit to be
Ansible 2.5.1 (default and latest version on Ubuntu 18.04 LTS).

As silly as it is, certain distributions ("LTS" even) are 13 bugfix
versions of Ansible behind.

From now on, we try to auto-detect buggy Ansible versions and tell the
user. We also provide some tips for how to upgrade Ansible or
run it from inside a Docker container.

My testing shows that Ansible 2.4.0 and 2.4.6 are OK.
All other intermediate 2.4.x versions haven't been tested, but we
trust they're OK too.

From the 2.5.x releases, only 2.5.0 and 2.5.1 seem to be affected.
Ansible 2.5.2 corrects the problem with `include_tasks` + `with_items`.

docs/prerequisites.md

@@ -4,7 +4,7 @@
 
 - [Python](https://www.python.org/) being installed on the server. Most distributions install Python by default, but some don't (e.g. Ubuntu 18.04) and require manual installation (something like `apt-get install python`).
 
-- the [Ansible](http://ansible.com/) program being installed on your own computer. It's used to run this playbook and configures your server for you. Version 2.4 or later is required (see `ansible --version`).
+- the [Ansible](http://ansible.com/) program being installed on your own computer. It's used to run this playbook and configures your server for you. Take a look at [our guide about Ansible](ansible.md) for version requirements or alternative ways to run Ansible.
 
 - properly configured DNS SRV record for `<your-domain>` (details in [Configuring DNS](configuring-dns.md#configuring-dns) below)