Decode traceback filenames with correct encoding

@mitsuhiko 's python-unio library actually assumes utf-8 on
Linux and BSD. I decided to commit this simple change instead because
the worst things that could happen would be:

1. Werkzeug's debugger crashes again on decoding, like it did before
2. The filenames are garbled and we end up with empty code blocks for
   some frames in the in-browser traceback.

None of these cases seems to be critical enough to justify the inclusion
of a complex stdlib wrapper like it's done in python-unio.

Fix #469