mediawiki: add a trusty check to apache envvars

When https://gerrit.wikimedia.org/r/#/c/147514/ is merged, we'll be able to use
apache::def here. Until then, this is an interim strategy for getting the
Trusty app servers on the beta cluster to run apache2 with -D HHVM.

Change-Id: I452910017b19721df65177aa3c79e4f72e0f621e