appveyor: fix Python 2 job

Update virtualenv so the tox environment use a more recent version
of pip (and probably setuptools/wheel too).

appveyor.yml

@@ -26,7 +26,7 @@ cache:
 test_script:
   - python --version
   - python -m pip install --disable-pip-version-check --upgrade pip setuptools wheel
-  - pip install --upgrade tox tox-venv
+  - pip install --upgrade tox tox-venv virtualenv
   - pip freeze --all
   - python bootstrap.py
   - tox -- --cov