Upgrade tox and virtualenv in appveyor to make sure we have the latest pip.

Signed-off-by: Ying <[email protected]>

Author: cyli

appveyor.yml

@@ -3,7 +3,7 @@ version: '{branch}-{build}'
 install:
   - "SET PATH=C:\\Python27-x64;C:\\Python27-x64\\Scripts;%PATH%"
   - "python --version"
-  - "pip install tox==2.1.1 virtualenv==13.1.2"
+  - "pip install tox==2.7.0 virtualenv==15.1.0"
 
 # Build the binary after tests
 build: false