Basemap Dependencies update in the documentation.

[HughP]

Basemap documentation currently says that PIL is required for some functions. However, PIL has largely been surpassed by Pillow. And in fact Pillow will work for basemap instead of PIL. So I recommend updating the documentation to reflect Pillow instead of PIL.

Optional libraries

PIL
Python Imaging Library (download), only needed for bluemarble(), etopo(), shadedrelief() and warpimage() instance methods.

[molinav]

I come from seven years later just to notify you that this part of the documentation has been updated. ;-)