[ticket/12320] No longer allow Travis CI HHVM environment to fail.

PHPBB3-12320
[ticket/12320] Install a nightly build of HHVM.

PHPBB3-12320
Merge remote-tracking branch 'bantu/ticket/12320' into develop-olympus

* bantu/ticket/12320:
  [ticket/12320] Install a nightly build of HHVM.
  [ticket/12320] No longer allow Travis CI HHVM environment to fail.
[ticket/12381] Fix broken error message when selecting invalid DB driver

PHPBB3-12381
Merge pull request #2279 from Elsensee/ticket/12381

[ticket/12381] Fix broken error message when selecting invalid DB driver

* Elsensee/ticket/12381:
  [ticket/12381] Fix broken error message when selecting invalid DB driver