[ticket/12592] Rename mysql environment to mysqli.

PHPBB3-12592
[ticket/12592] Add PHP 5.4 environment using mysql driver.

PHPBB3-12592
Merge pull request #2496 from bantu/ticket/12592

[ticket/12592] Run mysql driver on Travis CI.

* bantu/ticket/12592:
  [ticket/12592] Add PHP 5.4 environment using mysql driver.
  [ticket/12592] Rename mysql environment to mysqli.