Change categories when editing a rule

#165 
This function fails when trying to move an entire category under a new parent. Lets instead just use `change_parent` like this:

```php
	/**
	* Change rule parent
	*
	* @param int $rule_id The current rule identifier
	* @param int $new_parent_id The new rule parent identifier
	* @return null
	* @access public
	* @throws \phpbb\boardrules\exception\out_of_bounds
	*/
	public function change_parent($rule_id, $new_parent_id)
	{
		$rule_id = (int) $rule_id;
		$new_parent_id = (int) $new_parent_id;

		// Try to change rule parent
		try
		{
			$this->nestedset_rules->change_parent($rule_id, $new_parent_id);
		}
		catch (\OutOfBoundsException $e)
		{
			throw new \phpbb\boardrules\exception\out_of_bounds('rule_id');
		}
	}
```