Upgrade gulp version

js/package.json

@@ -11,7 +11,7 @@
     "glob": "~6.0.4",
     "google-closure-compiler": "~20190301.0.0",
     "google-closure-library": "~20190301.0.0",
-    "gulp": "~3.9.0",
+    "gulp": "~4.0.1",
     "jasmine": "~2.4.1"
   },
   "scripts": {