Explicitly clean cache and add force flag to all npm commands in AppVeyor

Author: darrenscerri

appveyor.yml

@@ -31,6 +31,7 @@ install:
   # TODO: Remove after https://github.com/appveyor/ci/issues/1426 is fixed
   - set PATH=C:\Program Files\Git\mingw64\bin;%PATH%
   - ps: Install-Product node $env:nodejs_version $env:platform
+  - alias npm="npm cache clean && npm --force"
 
 build: off