Use geodatasets for examples instead of geopandas

[ianthomas23]

There has been a change in the example data that is/isn't shipped with geopandas, in particular the various "naturalearth" datasets. We should replace these with use of other examples from geodatasets. The only place in datashader that this relevant is in 8_Polygon.ipynb. See holoviz/geoviews#635.

A suitable replacement is something like

import geodatasets as gds
import geopandas
us_counties = geopandas.read_file(gds.get_path('geoda health'), include_fields=["statename", "countyname", "cty_pop200"])

which works locally but gives some obscure projection error in CI. I don't wish to hold off the 0.15.0 release any longer, so this can be dealt with after that.