Gökçen Nurlu authored Jun 01, 2018 and Tristan Van Berkom committed Jun 05, 2018

The cleanup was supposed not to remove folders (`/dev`, `/tmp`, `/proc`) if
they already existed before bwrap but it did the opposite: it tried to remove
them if they existed before, and didn't remove them if they were created during
bwrap. This was caused by a `not` clause, and this removes it.

Fixes #379