Use /tmp as cache location to fix cross-rs builds

[rgwood]

A fix for the cross-rs build issue mentioned in #38. I figure /tmp is reasonable on any Unix system and we can fall back to cache_dir() for non-Unix, non-Windows systems?

[wrapperup]

Looks good, thanks! Made sure it worked on my end. I don't run anything more than Linux/Mac/Windows, so I think it's reasonable to only support windows and unix. If cache_dir doesn't work for other platforms, I can do it on an issue basis.