Fixes all failing tests on windows.

`test/test-usemin.js`: Ensures a clean state even if the test fails. 

`lib/revvedfinder.js`. One can argue if a windows user should use backslashes in the revmap. In my opinion only forwardslashes should be used in revmaps.

`test/test-revvedfinder.js`: Updates tests to refelect the changes made in `lib/revvedfinder.js`