ci(appveyor): bypass npm cache

node seems to have an issue where there are random permission denied errors on windows.

npm/npm#9696

Author: jimthedev

appveyor.yml

@@ -7,7 +7,7 @@ install:
   # Get the latest stable version of Node.js or io.js
   - ps: Install-Product node $env:nodejs_version
   # install modules
-  - npm install
+  - npm install --force
 
 # Post-install test scripts.
 test_script: