beta: clear config cache on mw-config update

The beta-mediawiki-config-update job introduced by 4e299a5 would simply
checkout to the new commit.  That is not enough to get the configuration
applied on apaches which monitor the timestamp of
InitialiseSettings.php. Touching the file is enough to clear the config
cache on the apaches instances.

Change-Id: Ib978b7fa63b5c4f4b379fd8a1fa6f98b521a8ad5