Repaired remaining subprocess calls for Windows users

Hello there Michael, hope you're doing well :smile: 

This pull requests completes the fix in https://github.com/miracle2k/webassets/pull/273/files across all remaining subprocess Popen calls.  I noticed this while trying to use coffeescript on Windows and realising that its subprocess call was missed, so I went through and did all the ones which were missed and repaired them.  I can at least confirm that coffeescript now works correctly on Windows.

Kindest Regards
Fotis