Add support for 'latest minor version' of d3

[npjc]

If I understand correctly r2d3 D3 support up to 5.0.0 but suggests that the latest minor version is supported (see screenshot below). How would one go about supporting up to the actual latest minor release (5.9.1 at the time of this writing, see d3/releases)?

This would make r2d3 better as it could take advantage of the latest improvements. As an example, the very useful+convenient d3-selection.join is >= 5.8.0.

I would be happy to help but I'm not sure how to implement this as I am not yet very familiar with the package. From my experiments It seems that simply adding/changing the minified source in www/inst/d3 doesn't cause an error but does cause the default d3 script to plot an empty svg.