fix virtualenv to not leave so much crap in /tmp

Each run of virtualenv leaves crap in /tmp/tmp\* - it unpacks distribute or something. It was recently changed to use tempfile instead of a hardcoded /tmp:
https://bitbucket.org/ianb/virtualenv/changeset/8be37c509fe5

...but it would be nice if it also cleaned up after it was done. This generates a lot of cruft while testing and will probably also do that in production.
