Include Python 2.7 in the appveyor matrix to capture failures on older Pythons. Ref #704.

appveyor.yml

@@ -2,6 +2,7 @@ environment:
 
   matrix:
     - PYTHON: "C:\\Python35-x64"
+    - PYTHON: "C:\\Python27-x64"
 
 install:
   # symlink python from a directory with a space