replace asserting doctrine is instance of Registry with assert it is proper interface

doctrine migration commands is showing warning when using custom implemetation of doctrine manager
this is why ive changed assert instance of Registry to assert instance of ManagerRegistry interface
as only one method getManagerNames is used that is declared in this interface
Fixes #400

Author: Slawomir Rostek

Command/Helper/DoctrineCommandHelper.php

@@ -5,9 +5,9 @@
 namespace Doctrine\Bundle\MigrationsBundle\Command\Helper;
 
 use Doctrine\Bundle\DoctrineBundle\Command\Proxy\DoctrineCommandHelper as BaseDoctrineCommandHelper;
-use Doctrine\Bundle\DoctrineBundle\Registry;
 use Doctrine\DBAL\Sharding\PoolingShardConnection;
 use Doctrine\DBAL\Tools\Console\Helper\ConnectionHelper;
+use Doctrine\Persistence\ManagerRegistry;
 use LogicException;
 use Symfony\Bundle\FrameworkBundle\Console\Application;
 use Symfony\Component\Console\Input\InputInterface;
@@ -27,7 +27,7 @@ public static function setApplicationHelper(Application $application, InputInter
     {
         $container = $application->getKernel()->getContainer();
         $doctrine  = $container->get('doctrine');
-        assert($doctrine instanceof Registry);
+        assert($doctrine instanceof ManagerRegistry);
 
         $managerNames = $doctrine->getManagerNames();