Add minimum version requirement for Ansible

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
+- 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`).
 
 - properly configured DNS SRV record for `<your-domain>` (details in [Configuring DNS](configuring-dns.md#configuring-dns) below)