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
Merge pull request #24 from stamen/stamen-forked

Use Stamen-forked Natural Earth