title: Value of type UIViewController has no member topViewController after Xcode 7 update
body: <p>After I updated to Xcode 7, I get this error "Value of type <code>UIViewController</code> has no member topViewController." Anyone else run into this? Is <code>topViewController</code> no longer a part of <code>UIViewController</code>?</p>
  
  <pre><code> override func viewDidLoad() {
      super.viewDidLoad()
      self.navigationItem.leftBarButtonItem = self.editButtonItem()
      let addButton = UIBarButtonItem(barButtonSystemItem: .Search,
          target: self, action: "addButtonPressed:")
      self.navigationItem.rightBarButtonItem = addButton
  
      if let split = self.splitViewController {
          let controllers = split.viewControllers
          self.detailViewController =
              controllers[controllers.count-1].topViewController as? ### Error heres
          DetailViewController
      }
  
      model = Model(delegate: self) // create the Model
      model.synchronize() // tell model to sync its data
      self.navigationController?.toolbarHidden = false;
      self.navigationItem.backBarButtonItem = UIBarButtonItem(title:"Home", style:.Plain, target:nil, action:nil)
  }
  </code></pre>