Fix tests on vagrant shared 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`.
:question: If this is a Virtualbox bug, create / find an issue at https://www.virtualbox.org/wiki/Bugtracker and link to it from here.