Mark RBLResizableImage.capInsets as atomic

Matches NSImage on 10.10.

Rebel/RBLResizableImage.h

@@ -18,6 +18,6 @@
 //
 // Any portion of the image not covered by end caps will be tiled when the image
 // is drawn.
-@property (nonatomic, assign) NSEdgeInsets capInsets;
+@property (assign) NSEdgeInsets capInsets;
 
 @end