Upgrade pip by calling via python. (#1067)

This is recommended in the appveyor error message.

appveyor.yml

@@ -32,7 +32,7 @@ install:
   # the parent CMD process).
   - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
 
-  - pip install --upgrade pip --user
+  - python -m pip install --upgrade pip --user
   - pip install pipenv
 
   # Find latest image magick version by parsing website