Securely install latest stable virtualenv

as per https://github.com/TheTorProject/ooni-probe/issues/263
Install latest stable virtualenv
Extract the packages to the temporary directory.
Use the latest versions of pip and virtualenv
Merge branch 'setup-virtualenv'

* setup-virtualenv:
  Use the latest versions of pip and virtualenv
  Extract the packages to the temporary directory.
  Install latest stable virtualenv
  Securely install latest stable virtualenv