gzip --force, to overwrite existing log files

We do not clean out /log/ between runs :/ So overwrite to keep the
artifacts.  Will have to figure out a way to properly clear the log
beforehand.

Jobs updated:

mediawiki-core-qunit
mwext-EventLogging-qunit
mwext-GuidedTour-qunit
mwext-MobileFrontend-qunit
mwext-VisualEditor-qunit
mwext-Wikibase-qunit

Change-Id: Ie1ae101b4d92727c558e998443d8190a041f7c92