title: Use of unresolved identifier 'kCAFillModeForwards'
body: <p>When I try to set <code>animation.fillMode = kCAFillModeForwards</code> Xcode can't compile and display an error. "<strong>Use of unresolved identifier 'kCAFillModeForwards'"</strong>.</p>
  
  <p>I've already used this in previous projects without any issues, did somebody already encountered this behaviour?</p>
  
  <pre><code>func animateGradient() {
          currentGradient += 1
          let animation = CABasicAnimation(keyPath: Animation.keyPath)
          animation.duration = animationDuration
          animation.toValue = currentGradientSet()
          animation.fillMode = kCAFillModeForwards
          animation.isRemovedOnCompletion = false
          animation.delegate = self
          gradient.add(animation, forKey: Animation.key)
  }
  </code></pre>