Update the vendored unicodecsv to fix None handling

The old version of unicodecsv incorrectly (according
https://docs.python.org/2/library/csv.html#csv.writer) encoding None
values as the string 'None', instead of the string '' as the python
documentation specifies.

The newest version of unicodecsv has fixed this.

Fixes #121
Merge pull request #155 from fusionbox/update-unicodecsv

Update the vendored unicodecsv to fix None handling