Transforms strong into weak reference for `rx_notification` wrapper.

RxCocoa/Common/Observables/NSNotificationCenter+Rx.swift

@@ -21,7 +21,7 @@ extension NSNotificationCenter {
     */
     @warn_unused_result(message="http://git.io/rxs.uo")
     public func rx_notification(name: String, object: AnyObject? = nil) -> Observable<NSNotification> {
-        return Observable.create { observer in
+        return Observable.create { [weak object] observer in
             let nsObserver = self.addObserverForName(name, object: object, queue: nil) { notification in
                 observer.on(.Next(notification))
             }