Fix an issue with trailing path separators in the --cache-dir argument.

[chrisgavin]

This fixes an issue where using a non-existent cache directory behaves differently depending on whether it has a trailing slash.

Before this change, using a trailing slash would always give an error because we check if the parent (as determined by filepath.Dir(...) exists before creating the cache directory. When a path has a trailing slash, filepath.Dir(...) returns the input path. This is a Go bug in my opinion.

To work around this issue, I've called filepath.Clean(...) on the path before we initialize the cache, which strips any trailing slashes. I've also added a test for this case.