This method is only available on newer versions.
The first available version (v2_6_3) is shown here.
download_cache_path(spec)
private
Returns the global cache
path of the calling Rubygems::Source object.
Note that the Source determines the path’s
subdirectory. We use this subdirectory in the global cache path so that gems with
the same name – and
possibly different versions – from different sources are saved to their
respective subdirectories and do not override one another.