Here is the cleanest way to achieve this in iOS 7.0 and above.
scrollView.keyboardDismissMode = UIScrollViewKeyboardDismissModeOnDrag;
Or
scrollView.keyboardDismissMode = UIScrollViewKeyboardDismissModeInteractive;
In Swift:
scrollView.keyboardDismissMode = .onDrag
Or
scrollView.keyboardDismissMode = .interactive