Run MediaWiki on Apache + fastcgi + HHVM

- The default is still to use the Zend interpreter.
- role::hhvm updated to install hhvm-nightly package from Facebook.
- Enable multiverse repos in apt module. Needed for libapache2-mod-fastcgi.
- When role::hhvm is enabled, Apache will switch to fastcgi + HHVM.
- HHVM uses apache::env type from I3b387fe6c to set 'HHVM' flag.
- Apache configuration files can branch on presence / absence of HHVM using
  <IfDefine HHVM> and <IfDefine !HHVM> blocks.
- Update MediaWiki's Apache configuration file to use this technique so that it
  is runnable under both HHVM and PHP, depending on which is present.

Change-Id: Ied0b9e5d969b2df5b8e7c744f5c36ef78bd9c024