Test integration/phpunit against mw core branches

Generate a job per maintained branch of MediaWiki core so we can assert
that changes proposed to integration/phpunit let us still run tests.

Creates:

integration-phpunit-mediawiki-REL1_22
integration-phpunit-mediawiki-REL1_23
integration-phpunit-mediawiki-REL1_24
integration-phpunit-mediawiki-master

Change-Id: I151285fb04aa148d847f6c0b4c6cc6c7ab8411fa