Serve from CloudFlare's CDNJS

Hi I :heart: your library and want to use it without needing to save a copy of its code on my hosted blog.

So I submitted it to be hosted in CDNJS and they accepted it!
https://github.com/cdnjs/cdnjs/pull/1721

The CDNJS maintainers asked that you tag the current version in git as `1.3`.

This pull request just updates the readme to point to where a copy of your library lives at https://cdnjs.cloudflare.com/ajax/libs/gist-embed/1.3/gist-embed.min.js