[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.