Upgraded to symfony 2.6
Loosened symfony version constraints
Merge pull request #5234 from GrahamCampbell/4.3-loosen-symfony

[4.3] Loosened Symfony Version Constraints
Revert "[4.3] Loosened Symfony Version Constraints"
Merge pull request #5231 from GrahamCampbell/4.3-symfony

[4.3] Upgraded To Symfony 2.6