should use protocol-relative url to load resources

As you see, we'll get warning on the demo page(maybe in the toolbar or in console), because the page was loaded via https but the resources were loaded via http protocol, which is not save, so may be blocked by client(browser), so it'll be better to use protocol-relative url to load resource here.

reference: http://www.paulirish.com/2010/the-protocol-relative-url/