Calling [super setImage] apparently doesn't set the layer contents…

This fixes an issue where setting the image one time works, but
setting it the next time seems to clear out the image.
I'm not sure this is the right solution. The issue is that displayLayer is returning self.currentFrame.CGImage. It should return self.image.CGImage. See https://github.com/Flipboard/FLAnimatedImage/pull/34