mediawiki: move testwiki to HAT

This patch will make mw1017 (the testwiki host) reinstall with trusty,
which will automagically make the mediawiki module use HHVM instead of
mod_php.

Change-Id: Idaf3afd1f230a7a3547a74a68709b1a3cfaa7430
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>