[ticket/12423] Increase Composer minimum-stability to stable.

PHPBB3-12423
[ticket/12423] Move PHP CodeSniffer from non-stable 1.5.0RC4 to latest 1.5.2.

PHPBB3-12423
[ticket/12424] Update Symfony component from v2.3.4 to v2.3.12.

PHPBB3-12424
Merge pull request #2334 from bantu/ticket/12423

Stable Composer Minimum Stability + Some Composer Dependency Updates

* bantu/ticket/12423:
  [ticket/12424] Update Symfony component from v2.3.4 to v2.3.12.
  [ticket/12423] Move PHP CodeSniffer from non-stable 1.5.0RC4 to latest 1.5.2.
  [ticket/12423] Increase Composer minimum-stability to stable.