'Cancel' for PromiseKit -- fix platform check for 'addObserver' usage

Author: dougzilla32

Sources/NSNotificationCenter+Promise.swift

@@ -38,11 +38,11 @@ extension NotificationCenter {
     /// Observe the named notification once
     public func cancellableObserve(once name: Notification.Name, object: Any? = nil) -> CancellablePromise<Notification> {
         let (promise, resolver) = CancellablePromise<Notification>.pending()
-#if !os(Linux)
-        let id = addObserver(forName: name, object: object, queue: nil, using: resolver.fulfill)
-#else
+      #if os(Linux) && ((swift(>=4.0) && !swift(>=4.0.1)) || (swift(>=3.0) && !swift(>=3.2.1)))
         let id = addObserver(forName: name, object: object, queue: nil, usingBlock: resolver.fulfill)
-#endif
+      #else
+        let id = addObserver(forName: name, object: object, queue: nil, using: resolver.fulfill)
+      #endif
 
         promise.appendCancellableTask(task: ObserverTask { self.removeObserver(id) }, reject: resolver.reject)