[Tests] test on node 7 on windows, too

Author: ljharb

appveyor.yml

@@ -3,6 +3,7 @@ environment:
     - nodejs_version: '4'
     - nodejs_version: '5'
     - nodejs_version: '6'
+    - nodejs_version: '7'
 install:
   - ps: Install-Product node $env:nodejs_version
   - set CI=true