Update external links on index page to use https

Important for http://webchat.freenode.net/ as it doesn't redirect automatically.

Author: alexandrnikitin

0.19/index.html

@@ -352,9 +352,9 @@ <h4>Community</h4>
             <ul>
             <li><em>About us</em> See <a href="about.html#people">authors</a> and <a href="developers/contributing.html">contributing</a></li>
             <li><em>More Machine Learning</em> Find <a href="related_projects.html">related projects</a></li>
-            <li><em>Questions?</em> See <a href="faq.html">FAQ</a> and <a href="http://stackoverflow.com/questions/tagged/scikit-learn">stackoverflow</a></li>
+            <li><em>Questions?</em> See <a href="faq.html">FAQ</a> and <a href="https://stackoverflow.com/questions/tagged/scikit-learn">stackoverflow</a></li>
             <li><em>Mailing list:</em> <a href="https://mail.python.org/mailman/listinfo/scikit-learn">[email protected]</a></li>
-            <li><em>IRC:</em> #scikit-learn @ <a href="http://webchat.freenode.net/">freenode</a></li>
+            <li><em>IRC:</em> #scikit-learn @ <a href="https://webchat.freenode.net/">freenode</a></li>
             </ul>
 
             <form target="_top" id="paypal-form" method="post" action="https://www.paypal.com/cgi-bin/webscr">
@@ -498,4 +498,4 @@ <h4>Who uses scikit-learn?</h4>
       })();
     </script>
   </body>
-</html>
+</html>