Patch/fix updated at existing pivot

The updated_at column should only be updated when using `withTimestamps();`

```php
Message::find(1)->users()->sync(array(1 => array('unread' => 1))); // Throws error
```

Column not found: 1054 Unknown column 'updated_at' in 'field list'
This will fix this issue and only update the updated_at field when `withTimestamps()` is used.

```php
Message::find(1)->users()->withTimestamps()->sync(array(1 => array('unread' => 1)));
```