Some more tweaks to our docs versioning and the version switcher. Main issue is a mismatch in how the version is put into a string: whether a "v" prefix is included or not. In the git tag we include a "v" prefix, but in folium.__version__ we don't have that prefix.

In the new situation:

• the git tag will include a v prefix
• the folder in gh-pages will include a v prefix
• all other places are without that prefix

This change is already live on the gh-pages branch, including the manually created docs for v0.18.0.