maxcdn shut down, so hardcoded references to maxcdn need to be replaced

[k5o]

You can see my fix here (uses jsdelivr instead of maxcdn): k5o@3a1f7fd

For more context: twitter/twemoji#580

I'll be using my fork in the meantime but I think a PR can be made (and perhaps allow a config option to set the base aka asset_host) for this ruby gem as well 👍 Thanks!