AppVeyor: clear cache when package.json changes.
AppVeyor: clear cache when package.json changes.

MartyIX/clean-css,jakubpawlowicz/clean-css,vlajos/clean-css,island205/clean-css,scottgonzalez/clean-css,shelsonjava/clean-css,GerHobbelt/clean-css,yomed/clean-css,stuht/clean-css