title: Firebase function signInWithCustomToken undefined with Node.js
body: <p>So I've created a custom token as such</p>
  
  <pre><code>const token = authRef.createCustomToken(options.userId, {
      app: appRootName,
      debug: !!options.debug
  });
  </code></pre>
  
  <p>And then attempt to sign it in as such</p>
  
  <pre><code>return authRef.signInWithCustomToken(token).then((user) =&gt; {
      return user;
  });
  </code></pre>
  
  <p>Custom creation token succeeds, but signInWithCustomToken spits out this error.</p>
  
  <pre><code>TypeError: authRef.signInWithCustomToken is not a function
  </code></pre>
  
  <p>This is Firebase 3 for nodejs and the service account has been set up. The app was properly initialized (or token generation wouldn't work in the first place), what's the issue here?</p>