Compatibility with Symfony 2.3

Following error:

PHP Fatal error:  Class FR3D\LdapBundle\Validator\UniqueValidator contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Symfony\Component\Validator\ConstraintValidatorInterface::validate) in ../vendor/fr3d/ldap-bundle/FR3D/LdapBundle/Validator/UniqueValidator.php on line 68

Corrected by changing class type to Abstract