website: github ribbon should be a https link

docs/_themes/attic/sidebarusefullinks.html

@@ -1,5 +1,5 @@
 <a href="https://github.com/jborg/attic"><img style="position: fixed; top: 0; right: 0; border: 0;"
-  src="http://s3.amazonaws.com/github/ribbons/forkme_right_gray_6d6d6d.png" alt="Fork me on GitHub"></a>
+  src="https://s3.amazonaws.com/github/ribbons/forkme_right_gray_6d6d6d.png" alt="Fork me on GitHub"></a>
 
 <h3>Useful Links</h3>
 <ul>