[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.
Merge branch 'develop-olympus' into develop-ascraeus

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