[ticket/12319] Backport Travis CI HHVM environment to develop-olympus.

PHPBB3-12319
Merge remote-tracking branch 'bantu/ticket/12319' into develop-olympus

* bantu/ticket/12319:
  [ticket/12319] Backport Travis CI HHVM environment to develop-olympus.
[ticket/12316] Add Build Status Icon for develop-ascraeus to README.md

PHPBB3-12316
Merge remote-tracking branch 'VSEphpbb/ticket/12316' into develop-olympus

* VSEphpbb/ticket/12316:
  [ticket/12316] Add Build Status Icon for develop-ascraeus to README.md
[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