Fix server error.

Author: tlaverdure

package.json

@@ -1,6 +1,6 @@
 {
   "name": "laravel-echo-server",
-  "version": "1.5.6",
+  "version": "1.5.7",
   "description": "Laravel Echo Node JS Server for Socket.io",
   "repository": {
     "type": "git",

src/server.ts

@@ -99,8 +99,8 @@ export class Server {
      */
     httpServer(secure: boolean) {
         this.express = express();
-        this.express.use(function(req, res, next) {
-            for(var header in this.options.headers) {
+        this.express.use((req, res, next) => {
+            for (var header in this.options.headers) {
                 res.setHeader(header, this.options.headers[header]);
             }
             next();