Rasterize collection view cell for scrolling performance

It seems a bit minimalistic, but these two lines do notably improve scrolling performance on iOS8 (see issue https://github.com/jessesquires/JSQMessagesViewController/issues/492).

As discussed with @FredericJacobs, we can rasterise JSQMessagesCollectionView cells' layer. If the ```shouldRasterize``` flag is set to ```YES```, ["the layer is rendered as a bitmap in its local coordinate space and then composited to the destination with any other content."](https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/CALayer_class/index.html#//apple_ref/occ/instp/CALayer/shouldRasterize). This allows for the scroll view to scroll smoothly without worrying about redrawing the cells all the time.

See also : SO tickets [1](https://stackoverflow.com/questions/16336772/uicollectionview-performance-updatevisiblecellsnow)  [2](https://stackoverflow.com/questions/12898259/uiscrollview-performance-tips)