Fallback to Symfony's default 'required' option in RatingType

Fallback to Symfony's default 'required' option if not has been set in 'config' of RatingType.

Otherwise it is a bit confusing if you set 'required' to false but nothing happens.

```php
<?php
// ...
public function buildForm(FormBuilder $builder, array $options)
{
    $builder
        // ...
        ->add('rating', 'genemu_jqueryrating', array(
            'required' => false, // now this is enough
            'configs' => array(
                'required' => false, // without the patch this line is needed
            )
        ))
}
```