Make ValidatorServiceProvider work with both 2.1 and 2.2 of symfony/validator
Make ValidatorServiceProvider work with both 2.1 and 2.2 of symfony/validator

Closes #584, #585.

CarsonF/Silex,mathroc/Silex,Tobion/Silex,GromNaN/Silex,jdreesen/Silex,peterkokot/Silex,CarsonF/Silex,akucherevskiy/developex_test,lokhman/Silex,akucherevskiy/developex_test,GromNaN/Silex,mmauri04/Silex,SpacePossum/Silex,Incognito/silex-fork,akucherevskiy/developex_test,beealone/Silex,fhinkel/Silex,GromNaN/Silex,Incognito/silex-fork,RayRutjes/Silex,romainneutron/Silex,cesarmarinhorj/Silex,silexphp/Silex,cgsmith/Silex,SanderKnape/Silex,beealone/Silex,arduanov/Silex,tcx1000/Silex,Tobion/Silex,romainneutron/Silex,JPry/Silex,plotek/Silex,cesarmarinhorj/Silex,romainneutron/Silex,Tobion/Silex,Xephi/Silex,gencer/Silex,mTorres/Silex,fhinkel/Silex,JPry/Silex,fabpot/Silex,arduanov/Silex,fabpot/Silex,naderman/Silex,lokhman/Silex,tcx1000/Silex,Xephi/Silex,mTorres/Silex,naderman/Silex,CarsonF/Silex,naderman/Silex,peterkokot/Silex,peterkokot/Silex,cgsmith/Silex,jdreesen/Silex,RayRutjes/Silex,mmauri04/Silex,SpacePossum/Silex,plotek/Silex,mathroc/Silex,SanderKnape/Silex