title: Swift 3 warning for dispatch async
body: <p>I have this code:</p>
  
  <pre><code>DispatchQueue.global(priority: DispatchQueue.GlobalQueuePriority.default).async {
                  let url = URL(string: itemImageURL )
                  let data = try? Data(contentsOf: url!)
                  if data != nil {
                      DispatchQueue.main.async{
                          cell.advImage!.image = UIImage(data: data!)
                      }
                  }
              }
  </code></pre>
  
  <p>I get this warning in Swift 3:</p>
  
  <blockquote>
    <p>'default' was deprecated in iOS 8.0: Use qos attributes instead</p>
  </blockquote>
  
  <p>on the first line.</p>
  
  <p>Haven't found yet a solution. Has anybody?</p>