[ticket/12259] Reduce the size of our test matrix

Only test MariaDB or PostgreSQL once on PHP 5.4. This reduces our build matrix
by 10 builds (over half).

PHPBB3-12259
Merge remote-tracking branch 'EXreaction/ticket/12259-olympus' into develop-olympus

* EXreaction/ticket/12259-olympus:
  [ticket/12259] Reduce the size of our test matrix
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/12259] Reduce the size of our test matrix

Conflicts:
	.travis.yml