Now only setting the operation queue's qualityOfService on iOS 8+.

Source/Alamofire.swift

@@ -763,9 +763,12 @@ public class Request {
             self.queue = {
                 let operationQueue = NSOperationQueue()
                 operationQueue.maxConcurrentOperationCount = 1
-                operationQueue.qualityOfService = NSQualityOfService.Utility
                 operationQueue.suspended = true
 
+                if operationQueue.respondsToSelector("qualityOfService") {
+                    operationQueue.qualityOfService = NSQualityOfService.Utility
+                }
+
                 return operationQueue
             }()
         }