Forward compatibility towards sf2.5, use the Interface as typehint instead

The use of Symfony\Component\Validator\ExecutionContextInterface is deprecated since version symfony 2.5, to be removed in Symfony 3.0. 

ExecutionContext* are moved to the Context sub-namespace, but as backwards compatibility kept the usage of ExecutionContextInterface inplace. So Using the ExecutionContextInterface circumvents the 
'Catchable Fatal Error: Argument 1 passed to Packagist\WebBundle\Entity\Package::isPackageUnique() must be an instance of Symfony\Component\Validator\ExecutionContext, instance of Symfony\Component\Validator\Context\LegacyExecutionContext'