Add support for nesting within LayerGroup component

[snario]

This adds support for LayerGroup from react-leaflet so you can nest this as such:

<LayerGroup>
    <GeoJsonCluster data={ data }/>
</LayerGroup>

Before it would break whenever componentDidUpdate was called because the references within this.props.layerGroup._layers would not be linked properly.

Requires a re-build of the lib directory.