[ticket/12318] Correctly setup HHVM for functional tests.

PHPBB3-12318
[ticket/12318] Upgrade to a recent stable version of HHVM.

PHPBB3-12318
[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 remote-tracking branch 'bantu/ticket/12318-olympus' into develop-olympus

* bantu/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.
  [ticket/12318] Upgrade to a recent stable version of HHVM.
  [ticket/12318] Correctly setup HHVM for functional tests.