update deprecated code

when using berksfile in latest chef development kit we get:
DEPRECATED: Your Berksfile contains a site location pointing to the Opscode Community Site (site :opscode). Site locations have been replaced by the source location. Change this to: 'source "https://supermarket.getchef.com"' to remove this warning. For more information visit https://github.com/berkshelf/berkshelf/wiki/deprecated-locations