Merge pull request #196 from yanokwa/fix-appveyor

Fix appveyor build

appveyor.yml

@@ -26,7 +26,7 @@ install:
 
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - ps: pip install --disable-pip-version-check --user --upgrade pip
+  - ps: python -m pip install -U pip
 
   # We need wheel installed to build wheels
   - ps: pip install wheel