Use ASDisplayNodeScreenScale().

ASImageNode and ASHighlightOverlayLayer were both using
[[UIScreen mainScreen] scale] directly.  Bad.  No cookie.  Use
ASDisplayNodeScreenScale() instead.

(Also, regenerate Xcode project with Xcode 6 and current CocoaPods.)

AsyncDisplayKit/Private/ASDisplayNodeInternal.h

@@ -19,6 +19,7 @@
 #import "ASThread.h"
 
 BOOL ASDisplayNodeSubclassOverridesSelector(Class subclass, SEL selector);
+CGFloat ASDisplayNodeScreenScale();
 
 @class _ASPendingState;