Replace concrete ConstraintValidatorFactory type hint with interface

With symfony 2.5 a [LegacyConstraintValidatorFactory](https://github.com/symfony/Validator/blob/master/LegacyConstraintValidatorFactory.php) is used as argument if the 2.4 API is used.

Using the corresponding interface as type hint fix a fatal error.

This PR is against the 2.2 branch, is it still maintained ?