Securely install latest stable virtualenv

as per https://github.com/TheTorProject/ooni-probe/issues/263
Install latest stable virtualenv
Corrected path to decks directory given in README.md
Merge pull request #268 from tomwills/patch-1

Corrected path to decks directory given in README.md
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
Fix the unittests for the oonibclient.
Use the debian package instead of setup-dependencies
Add tests for the oonib reporting API
Merge pull request #271 from TheTorProject/fix/vagrantfile

Use the debian package instead of setup-dependencies
Merge pull request #272 from TheTorProject/fix/oonibclient

Fix/oonibclient