Implemented ConstraintValidator::validate()

Since version 2.1 of symfony/validator, ConstraintValidator::isValid()
is deprecated an triggers an E_DEPRECATED error.

Unfortunately the error handling in some circumstances triggers
SIGSEGV in PHP-FPM (I havn't been able to pinpoint the exact cause).

Now isValid() is just a BC wrapper for validate() not triggering any
errors.
Merge pull request #1299 from tPl0ch/hotfix/constraint-validator-deprecated

Implemented ConstraintValidator::validate()