Added use Doctrine\ODM\MongoDB\LoggableCursor; in DocumentPersister

Build passes, but it's missing a use statement.

FatalErrorException: Error: Class 'Doctrine\ODM\MongoDB\Persisters\LoggableCursor' not found in /vendor/doctrine/mongodb-odm/lib/Doctrine/ODM/MongoDB/Persisters/DocumentPersister.php line 429

Added missing Doctrine\ODM\MongoDB\LoggableCursor