Invalidate backing layer for <Group> on child updates.

Fixes Flipboard#25

Author: Michael Johnston

lib/Group.js

@@ -23,6 +23,7 @@ var Group = createComponent('Group', LayerMixin, ContainerMixin, {
  this.applyLayerProps(prevProps, props);
  this.updateChildren(props.children, transaction, context);
  this._currentElement = nextComponent;
+ this.node.invalidateLayout();
  },
 
  unmountComponent: function () {