Add Node 8 to Appveyor tests

appveyor.yml

@@ -5,13 +5,14 @@ environment:
   - nodejs_version: "4"
   - nodejs_version: "6"
   - nodejs_version: "7"
+  - nodejs_version: "8"
 
 # Install scripts. (runs after repo cloning)
 install:
   # Get the latest stable version of Node.js or io.js
   - ps: Install-Product node $env:nodejs_version
   # install modules
-  - npm cache clean
+  - npm cache clean --force
   - npm set registry http://registry.npmjs.org/
   - npm install