Fixes uncaught RuntimeException 'Property "iid" does not exist

Fatal error:  Uncaught exception 'Gitlab\Exception\RuntimeException' with message 'Property "iid" does not exist for Gitlab\Model\MergeRequest object' in lib\Gitlab\Model\AbstractModel.php:50
Adds the new property "iid" to the MergeRequest Model
Adapting composer.json for Packagist