Update appveyor.yml.
Update appveyor.yml.

Use variables for cache items and remove the unneeded grunt-cli global installation since it's in devDependencies.

paladox/grunt-contrib-qunit,gruntjs/grunt-contrib-qunit,labertasch/grunt-contrib-qunit,labertasch/grunt-contrib-qunit,gruntjs/grunt-contrib-qunit,paladox/grunt-contrib-qunit