[ticket/11920] Add MariaDB environment to Travis-CI.

PHPBB3-11920
[ticket/11920] Have to use empty root password otherwise db creation fails.

PHPBB3-11920
Merge pull request #1780 from bantu/ticket/11920

[ticket/11920] Add MariaDB environment to Travis-CI.
Merge remote-tracking branch 'github-phpbb/develop-olympus' into develop

* github-phpbb/develop-olympus:
  [ticket/11920] Have to use empty root password otherwise db creation fails.
  [ticket/11920] Add MariaDB environment to Travis-CI.

Conflicts:
	.travis.yml
[develop] Update version to 3.1.0-a2-dev