update firebase SDK version in service worker (#353)

messaging/firebase-messaging-sw.js

@@ -1,8 +1,8 @@
 // Import and configure the Firebase SDK
 // These scripts are made available when the app is served or deployed on Firebase Hosting
 // If you do not serve/host your project using Firebase Hosting see https://firebase.google.com/docs/web/setup
-importScripts('/__/firebase/5.5.6/firebase-app.js');
-importScripts('/__/firebase/5.5.6/firebase-messaging.js');
+importScripts('/__/firebase/6.3.4/firebase-app.js');
+importScripts('/__/firebase/6.3.4/firebase-messaging.js');
 importScripts('/__/firebase/init.js');
 
 var messaging = firebase.messaging();
@@ -15,8 +15,8 @@ var messaging = firebase.messaging();
  // Give the service worker access to Firebase Messaging.
  // Note that you can only use Firebase Messaging here, other Firebase libraries
  // are not available in the service worker.
- importScripts('https://www.gstatic.com/firebasejs/4.8.1/firebase-app.js');
- importScripts('https://www.gstatic.com/firebasejs/4.8.1/firebase-messaging.js');
+ importScripts('https://www.gstatic.com/firebasejs/6.3.4/firebase-app.js');
+ importScripts('https://www.gstatic.com/firebasejs/6.3.4/firebase-messaging.js');
 
  // Initialize the Firebase app in the service worker by passing in the
  // messagingSenderId.