Use protocol relative URL for Twitter

.themes/classic/source/_includes/twitter_sharing.html

@@ -4,7 +4,7 @@
       var twitterWidgets = document.createElement('script');
       twitterWidgets.type = 'text/javascript';
       twitterWidgets.async = true;
-      twitterWidgets.src = 'http://platform.twitter.com/widgets.js';
+      twitterWidgets.src = '//platform.twitter.com/widgets.js';
       document.getElementsByTagName('head')[0].appendChild(twitterWidgets);
     })();
   </script>