Update layout.njk

Author: yosuke-furukawa

source/layout/layout.njk

@@ -38,7 +38,7 @@
           <ul>
             <li><small><a target="_blank" href="https://github.com/nodejsjp/nodejsjp.github.com">GitHub</a></small></li>
             <li><small><a target="_blank" href="https://twitter.com/nodejsjp">Twitter</a></small></li>
-            <li><small><a target="_blank" href="https://iojs-jp-slack.herokuapp.com/">Slack</a></small></li>
+            <li><small><a target="_blank" href="https://join.slack.com/t/jsconfjp/shared_invite/zt-1j7fqgvbq-Qa334TtwwKWC1eJBrgJ3yQ">Slack</a></small></li>
           </ul>
         </nav>
       </article>