Fix CRToastInteractionResponder nil block crash.

Passing a nil block into the CRToastInteractionResponder convenience method would cause a crash when the gesture recognizer was fired. I have added checks to make sure the block exists before calling it. 

Since the toast has an option to automatically dismiss, it is possible that there doesn't need to be a completion block.

I also considered making a second class method without the block parameter, as per below. I've left that out for now, but I could add it in if it's agreed to be useful. 

```obj-c
+ (instancetype)interactionResponderWithInteractionType:(CRToastInteractionType)interactionType 
                                   automaticallyDismiss:(BOOL)automaticallyDismiss;
```