Move MediaWiki HHVM job to Trusty instance(s)

Updates non voting jobs:

    mediawiki-core-regression-hhvm-master
    mediawiki-core-regression-hhvm-REL1_19
    mediawiki-core-regression-hhvm-REL1_20
    mediawiki-core-regression-hhvm-REL1_21
    mediawiki-core-regression-hhvm-REL1_22
    mediawiki-core-regression-hhvm-REL1_23

Change-Id: Iba535ac0e5e462309557d5c7163600acc90cfc83