createCustomToken returns a Promise

Same as https://github.com/firebase/functions-samples/pull/84 but for LinkedIn Auth.

linkedin-auth/functions/index.js

@@ -145,8 +145,9 @@ function createFirebaseAccount(linkedinID, displayName, photoURL, email, accessT
   // 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.
-    const token = admin.auth().createCustomToken(uid);
-    console.log('Created Custom token for UID "', uid, '" Token:', token);
-    return token;
+    return admin.auth().createCustomToken(uid).then((token) => {
+      console.log('Created Custom token for UID "', uid, '" Token:', token);
+      return token;
+    });
   });
 }