[ticket/11951] Add MariaDB to supported DBMSes in requirement section.

PHPBB3-11951
[ticket/11951] Add MariaDB to platforms we test on.

PHPBB3-11951
Merge remote-tracking branch 'github-bantu/ticket/11951' into develop-olympus

* github-bantu/ticket/11951:
  [ticket/11951] Add MariaDB to platforms we test on.
  [ticket/11951] Add MariaDB to supported DBMSes in requirement section.
[task/github-readme/travis] Fix Travis-Badge and link

PHPBB3-11968
Merge branch 'task/github-readme/travis' into develop-olympus

* task/github-readme/travis:
  [task/github-readme/travis] Fix Travis-Badge and link
[ticket/11920] Pin MariaDB repository to avoid conflicts with Ubuntu MySQL.

PHPBB3-11920
Merge remote-tracking branch 'bantu/ticket/11920' into develop-olympus

* bantu/ticket/11920:
  [ticket/11920] Pin MariaDB repository to avoid conflicts with Ubuntu MySQL.