[ticket/13215] Update Symfony components to v2.3.21.

PHPBB3-13215
Merge pull request #3062 from bantu/ticket/13215

[ticket/13215] Update Symfony components to v2.3.21.
Merge branch 'prep-release-3.1.0' into develop-ascraeus

* prep-release-3.1.0:
  [ticket/13215] Update Symfony components to v2.3.21.