Reveal 1.6: Crash when serialising `accessibilityLabel` containing an `NSAttributedString`

In Reveal 1.6, the Reveal library crashes the host iOS application when it encounters an accessibilityLabel property that contains an NSAttributedString. The API contract within UIAccessibility.h states that this property should contain an NSString value, however Apple have recommended putting an attributed string into this property at a previous WWDC (WWDC 2012 Session 210: https://developer.apple.com/videos/play/wwdc2012/210/).

The workaround until this is addressed is to use an NSString instead of an NSAttributedString in an object's accessibilityLabel.