use https to get favicon.ico

Even better would be to say

``href="//www.anthrocon.org/sites/all/themes/anthrocon-2015/images/anthrocon-2015/favicon.gif"``

saying ``//some-url`` will make it `http`` if the page containing the href is ``http`` and will make it ``https`` if the page containing it is ``https``.

At this point, I think Uber is just ALWAYS ``https`` so this should be fine, but I figured I'd mention that syntax since I'd been building websites for more than 10 years before I learned that trick :)