update for Swift 1.2

Author: mattneub

bk1ch13p380kvo/ch13p355kvo/AppDelegate.swift

@@ -45,7 +45,7 @@ class AppDelegate : UIResponder, UIApplicationDelegate {
         objectB = MyClass2()
         let opts : NSKeyValueObservingOptions = .New | .Old
         objectA.addObserver(objectB, forKeyPath: "value", options: opts, context: &con)
-        (objectA as MyClass1).value = true
+        (objectA as! MyClass1).value = true
         // comment out next line if you wish to crash
         objectA.removeObserver(objectB, forKeyPath: "value")
         objectA = nil