Update file_history.py

Fixed the bug that was added with last save (sorry).  The new code will work in both ST2 and ST3 and will try to migrate your project file history from using the old hash to the new project file name.  

This is not quite perfect (in my mind) because there can be more than one workspace per project , and it would be nice to have a per-workspace history (rather than a per-project history).  Unfortunately there aren't yet any workspace related functions in the api.