Let AbuseFilter work with 1.19

This patch allows the current AbuseFilter to work with 1.19, by only
using MWTimestamp if MediaWiki is version 1.20 or later.

Api still needs fixing, but this will get filters running.

Change-Id: I7c9eaf777d529ce15dfd85761784f9ad1443f2bf
Merge "Let AbuseFilter work with 1.19"