Using https to avoid the non-secure warning 

Hi guys!

As an example: https://joind.in/event/view/2166 - it gives you the "non-secure" warning - i.e. the page includes some non-https assets. We could probably also use the `//` protocol so that it matches the protocol used by the visitor on the site.

I *did* verify that the https `tile.openstreemap.org` URLs work identically to the `http` version. This seems like a simple enough change, but I did *not* actually get the code working locally to verify.

Thanks!