[ticket/12417] Revert "No longer allow Travis CI HHVM environment to fail."

hhvm-nightly 2014.04.16~precise introduced "Attempted to serialize
unserializable builtin class DOMElement" errors when running functional tests.

This reverts commit ff7cc77f9db66b7fddacd77f215d85cf0243b2fa.

PHPBB3-12417
Merge pull request #2324 from bantu/ticket/12417

[ticket/12417] Revert "No longer allow Travis CI HHVM environment to fai...

* bantu/ticket/12417:
  [ticket/12417] Revert "No longer allow Travis CI HHVM environment to fail."