Change minimum version to one that supports course_header

Hi, great theme, but it doesn't work on versions of Moodle before this change to outputrenderers.php:

https://github.com/moodle/moodle/commit/fdd4b9a50ad78de738eaf659ec769dcd55d643a4#lib/outputrenderers.php

Here's a patch to add a minimum version number that supports it.

Cheers

Michael