Revert "Indicate that Ubuntu Bionic (18.04) is not supported yet"

This reverts commit 8d774db3bc.

Docker is released in the Docker CE stable repository now.

Additionally, it's version 18.03, which doesn't suffer
any of the problems we've observed with 18.05 (edge/nightly).

README.md

@@ -50,7 +50,7 @@ This is similar to the [EMnify/matrix-synapse-auto-deploy](https://github.com/EM
 
 ## Prerequisites
 
-- **CentOS** (7.0+), **Debian** (9/Stretch+) or **Ubuntu** (anything 16.04+, except 18.04 for now) server. This playbook can take over your whole server or co-exist with other services that you have there.
+- **CentOS** (7.0+), **Debian** (9/Stretch+) or **Ubuntu** (16.04+) server. This playbook can take over your whole server or co-exist with other services that you have there.
 
 - the [Ansible](http://ansible.com/) program, which is used to run this playbook and configures everything for you