CSS background-image is not pointing to CDN endpoint in production mode

[deepaksethi]

On
https://github.com/niftylettuce/express-cdn/blob/master/lib/main.js#L188

String.replace returns a new string and hence the above line is effectively a NOOP
Furthermore the replace function is returning -- attribute+':url('+url+')'; -- irrespective of options.production.

The same issue will affect the font etc. too