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.

This fixes https://github.com/sonata-project/SonataAdminBundle/issues/1282