Update appveyor.yml.
Update appveyor.yml.

Use variables for cache items.

skamenetskiy/grunt-contrib-handlebars,kaidoe/grunt-contrib-handlebars,andrewhosgood/grunt-contrib-handlebars,szarouski/grunt-contrib-handlebars,rally25rs/grunt-contrib-handlebars,gruntjs/grunt-contrib-handlebars