Show dismiss button only when presented modally

* Modify the configuration of dismiss button so that it only appears when the controller is root view controller and presented modally.

I notice that there were [discussions](https://github.com/vtourraine/VTAcknowledgementsViewController/pull/13#commitcomment-5544466) about `navigationController.viewControllers` count being 0 in `viewDidLoad:`. I've tested in storyboard and it works fine with the configuration in `viewDidLoad:`. Not sure if there are other circumstances that would cause the count to be 0.