I upgraded the wrong pip

We need the pip in the virtualenv, which gets installed from
virtualenv's own bundled copy.  So we need a newer virtualenv with a
newer bundled pip.

appveyor.yml

@@ -18,7 +18,7 @@ init:
 install:
   - "set PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
   - python --version
-  - python -m pip install -U pip
+  - pip install -U virtualenv
   - pip install tox
 
 build: off