Make HHVM the default PHP on MediaWiki-Vagrant

* Replace role::hhvm with role::zend
* Replace 'HHVM' Apache parameter with 'ZEND'
* Some incidental cleanup:
** Don't install git-maintainers ppa; Trusty's version is sufficiently
   up-to-date.

Change-Id: I1fb84f51a4f94f8adbc1785eb122dccd347a46ef