Changed slack invite link.

index.html

@@ -56,7 +56,7 @@
 						<li><a href="https://stackoverflow.com/questions/tagged/three.js">questions</a></li>
 						<li><a href="https://discourse.threejs.org/">forum</a></li>
 						<li><a href="http://webchat.freenode.net/?channels=three.js">irc</a></li>
-						<li><a href="https://threejs-slack.herokuapp.com">slack</a></li>
+						<li><a href="https://join.slack.com/t/threejs/shared_invite/enQtMzYxMzczODM2OTgxLTQ1YmY4YTQxOTFjNDAzYmQ4NjU2YzRhNzliY2RiNDEyYjU2MjhhODgyYWQ5Y2MyZTU3MWNkOGVmOGRhOTQzYTk">slack</a></li>
 						<li><a href="https://twitter.com/threejs_org">twitter</a></li>
 					</ul>