[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 branch 'ticket/12384' into ticket/12384-ascraeus

* ticket/12384:
  [ticket/12384] Run Travis CI HHVM tests against MySQLi instead of MySQL.
[ticket/12384] Use mysqli on hhvm in ascraeus. Keep explicit matrix though.

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.
Merge pull request #2292 from bantu/ticket/12384-ascraeus

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

* bantu/ticket/12384-ascraeus:
  [ticket/12384] Use mysqli on hhvm in ascraeus. Keep explicit matrix though.
  [ticket/12384] Run Travis CI HHVM tests against MySQLi instead of MySQL.
Merge branch 'develop-olympus' into develop-ascraeus

* develop-olympus: