[Math] Enabled on the WMF cluster since ages.

Spotted by Mormegil
https://translatewiki.net/wiki/Thread:Support/The_Math_extension_not_listed_under_%E2%80%9CExtensions_used_by_Wikimedia%E2%80%9D

Change-Id: I8b963f120526bf8d6c80f6de2f7d40819086c9f6