Fixes #4706

• Removes the old non functional caching system for figures in doc/pyplots
• Remove the empty figure fallback when building the basemap figure
• Install Basemap when building the figures on Travis.
• PEP8 cleanup in basemap example

Basemap is now a dependency in the same way as any other dependencies (colorspacious, pillow ...) To make it harder to miss dependencies I have reversed the logic for warnings as errors so that it is default on.