PHP requirement is 7.2 (#42)

The current install of the server fails with PHP 7.1, it requires 7.2.

I'm not sure if that applies to Symfony 4 too, or if it's only this server. The Symfony doc says 7.1 is enough: https://symfony.com/doc/current/setup.html

Author: glye

docs/topics/prerequisites.md

@@ -2,11 +2,11 @@
 
 In order to run the server, you'll need a server which
 
-* is capable of running Symfony 4. This means: PHP 7.1
+* is capable of running Symfony 4. This means: PHP 7.2
 * has git installed
 * has Composer installed
 * is able to load the repos. That means: Access to the internet is needed, also some free disk space.
 
 It is recommended to setup some sort of monitoring. We use [Sentry](https://sentry.io).
 
-SSH access is useful to run [commands](commands.md) on the server. 
+SSH access is useful to run [commands](commands.md) on the server.