remove obsolete cdn links
remove obsolete cdn links

and provide instructions for retrieving up-to-date links from cdnjs

aaronhoffman/dc.js,aaronhoffman/dc.js,mtraynham/dc.js,dc-js/dc.js,mtraynham/dc.js,dc-js/dc.js,mtraynham/dc.js,aaronhoffman/dc.js,dc-js/dc.js,aaronhoffman/dc.js,mtraynham/dc.js,mtraynham/dc.js,aaronhoffman/dc.js,dc-js/dc.js,dc-js/dc.js