Stop Socket.IO-Server on ARSnovaSocketIOServer destroy

src/main/java/de/thm/arsnova/socket/ARSnovaSocketIOServer.java

@@ -70,6 +70,7 @@ public class ARSnovaSocketIOServer {
 		for (SocketIOClient c : server.getAllClients()) {
 			c.disconnect();
 		}
+		server.stop();
 	}
 
 	public void startServer() throws Exception {