Fix tests on vagrant sharfix-tests-on-vagrant-111ed folder.

Fixes #111.

It would be good to also have instructions on how to run the tests so that the temporary directory isn't a vagrant shared folder, but I'm not sure how to tell tox to put the temporary directory somewhere other than `.tox`.
It isn't clear to me that this is a bug. I think, rather, that it just a limitation of how virtualbox works. I don't think that I want the permissions of the group or everybody on the host machine to change just because I make a change in the VM. So virtualbox doesn't have a way of storing changes made to those permissions separately from user permissions.