Change modestmaps.min.js URL to be protocol relative

I found the Cloudfront URL with a `dig code.modestmaps.com`. I get that it may not be so good to hardcode a Cloudfront URL, but the Cloudfront distribution isn't SSL enabled and I can't load preview.html in Chrome because modestmaps.min.js is loaded over HTTP. (FYI, [Cloudfront SSL with SNI only has been free since March 2014](http://aws.amazon.com/about-aws/whats-new/2014/03/05/amazon-cloudront-announces-sni-custom-ssl/))