Fix Mixed Content issue when browsing over HTTPS

I use the *HTTPS Everywhere* extension to automatically browse over HTTPS when available. However, https://mojotech.github.io/jeet/ does not render nicely.

jQuery and fonts are loaded over HTTP (`http://`) which is a problem since mixed active content is now blocked by default on modern browsers. (see [Mixed Content on developer.mozilla.org](https://developer.mozilla.org/en-US/docs/Security/MixedContent))

This PR fixes the issue using the protocol-relative `//`.