Make SKScene fill the entire screen with Swift

I have the following in GameViewController.swift

class GameViewController: UIViewController {

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        println(self.view.frame.size)
        var skView:SKView = self.view as SKView
        if skView.scene == nil {
            skView.showsFPS = false
            skView.showsNodeCount = false

            // Create and configure the scene.
            var scene : SKScene = GameScene(size: skView.bounds.size)
            scene.scaleMode = SKSceneScaleMode.AspectFill

            // Present the scene.
            skView.presentScene(scene)
        }
    }

    override func prefersStatusBarHidden() -> Bool {
        return true
    }
}

      

        
        
        
      

    

it prints:

(320.0,480.0)

      

        
        
        
      

    

How do I GameScene

fill the entire screen and not leave black bars at the top and bottom of the screen?

code in GameScene.swift

class GameScene: SKScene {
    override func didMoveToView(view: SKView) {
        /* Setup your scene here */
        var background : SKSpriteNode = SKSpriteNode (imageNamed: "background.png")
        background.position = CGPointMake(self.frame.size.width/2, self.frame.size.height/2)
        self.addChild(background)
    }
}

      

        
        
        
      

    

background.png

is a size image 640x1138

Deployment target - iOS 7.1

I am using Xcode 6.0.1 (beta)