[ticket/12384] Run Travis CI HHVM tests against MySQLi instead of MySQL.

HHVM MySQLi has been fixed and works for us now.

PHPBB3-12384
Merge pull request #2282 from bantu/ticket/12384

[ticket/12384] Run Travis CI HHVM tests against MySQLi instead of MySQL.

* bantu/ticket/12384:
  [ticket/12384] Run Travis CI HHVM tests against MySQLi instead of MySQL.