Remove trailing comma in webpack config to support Node@<8

See https://gitlab.com/gitlab-org/gitlab-ce/issues/43669

config/webpack.config.js

@@ -362,7 +362,7 @@ if (IS_DEV_SERVER) {
           callback();
         })
       },
-    },
+    }
   );
   if (DEV_SERVER_LIVERELOAD) {
     config.plugins.push(new webpack.HotModuleReplacementPlugin());