Replace / with _ when caching
Replace / with _ when caching

This prevents pickle from getting confused about directories for leagues like 'OneWeekHCRampage/Beyond'.
Specifically it fixes `IOError: [Errno 2] No such file or directory: '.ladder_cache/OneWeekHCRampage/Beyond'`.

willroberts/pathofexile,willroberts/pathofexile,willroberts/pathofexile,willroberts/pathofexile