wp menu item add-post --parent-id=ID does not set menu item parent id

It seams it is missing the parent-id assoc args when trying to insert the menu item 

```shell
$ wp --info
PHP binary:	/usr/bin/php5
PHP version:	5.5.11-1~dotdeb.1
php.ini used:	/etc/php5/cli/php.ini
WP-CLI root dir:	phar://wp-cli.phar
WP-CLI global config:	
WP-CLI project config:	/var/www/wp/wp-cli.local.yml
WP-CLI version:	0.15.0
```