[ticket/12318] Add PPA providing dependencies for recent HHVM on Ubuntu 12.04.

PHPBB3-12318
[ticket/12318] Always install new config file on HHVM upgrade conflict.

PHPBB3-12318
[ticket/12318] Pin HHVM version to 3.0.0~precise.

PHPBB3-12318
[ticket/12318] Use MySQL instead of MySQLi for now on HHVM.

PHPBB3-12318
Merge branch 'ticket/12318-olympus' into ticket/12318

* ticket/12318-olympus:
  [ticket/12318] Use MySQL instead of MySQLi for now on HHVM.
  [ticket/12318] Pin HHVM version to 3.0.0~precise.
  [ticket/12318] Always install new config file on HHVM upgrade conflict.
  [ticket/12318] Add PPA providing dependencies for recent HHVM on Ubuntu 12.04.