Play nice with HTTPS installations

When running Redmine on HTTPS the following error will pop up in the Firebug console:

> Blocked loading mixed active content "http://fonts.googleapis.com/css?family=Lato:300,400,700"

This can be fixed by replacing `http://` with `//`, the protocol relative URL. See fe. http://www.paulirish.com/2010/the-protocol-relative-url/