Use Stamen-forked Natural Earth

Fetches the following updated archives from GitHub:

* ne_10m_geography_marine_polys.zip
* ne_50m_geography_marine_polys.zip
* ne_110m_geography_marine_polys.zip
* ne_10m_airports.zip

Using `data/ne-stamen/...` in place of `data/ne/...` will fetch versions from GitHub.

/cc @almccon