inject $wgWikimediaJenkinsCI = true

That is an horrrrible hack to let extensions (such as Wikibase) behave
differently when being run on Wikimedia Jenkins CI.  That is more or
less needed when running Wikibase under Apache for QUnit, since the
Jenkins environnement variables are not available to the Apache process,
the Wikibase entry point had no real way to detect where it was running
(currently uses JOB_NAME env variable, which is not set).

Change-Id: Ice52170ee507a34d15316580ad6be3ba83f01a04