Update index.js

Use arrow function

instagram-auth/functions/index.js

@@ -147,7 +147,7 @@ function createFirebaseAccount(instagramID, displayName, photoURL, accessToken)
   // Wait for all async task to complete then generate and return a custom auth token.
   return Promise.all([userCreationTask, databaseTask]).then(() => {
     // Create a Firebase custom auth token.
-    return admin.auth().createCustomToken(uid).then(function (token) {
+    return admin.auth().createCustomToken(uid).then((token) => {
       console.log('Created Custom token for UID "', uid, '" Token:', token);
       return token;
     });