[TASK] Update Symfony components to current LTS version

Allow the current Symfony LTS version for components. 2.5 is out of
date and 2.6 will be soon.

Update composer.lock for all symfony components to be consistent.

Update our code a bit to not trigger deprecation errors,

Change-Id: Ie4f495a95d8c4f9647e95ff611689f1bd5ed1208
Resolves: #68688
Releases: master
Reviewed-on: http://review.typo3.org/42203


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Patrik Karisch <p.karisch@pixelart.at>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>