Fixed bug where dismissing a presentedViewController would force RNSwipeViewController to unexpectedly revert to its centerViewController

[MaxwellPayne]

Ran into an issue where, after running presentViewController and subsequently dismissViewControlller from RNSwipeViewController's rightViewController, the RNSwipeViewController would force itself back to displaying centerViewController instead of sticking with the rightViewController. RNSwipeViewController will now suppress its viewWillAppear method if a presentedViewController is in the process of being dismissed.