Add isRequired to the array nodes in configuration

Now when I register the bundle and I do not set the configuration for `payment_success`, `payment_cancel`, `payment_fail` and `payment_process` Symfony will throw a ContextErrorException saying `Undefined index: payment_success` adding `isRequired` will not avoid the exception but the message will be more according with the configuration error.
Also @sfblaauw, it's not in your PR but can you remove 200 of the created ``Response`` object in the end of the file ?

An ``Response`` object returns 200 as status code if not updated.