title: Why is using accounts.ui.config crashing my app?
body: <p>Code (as suggested in the <a href="http://docs.meteor.com/#accounts_ui_config" rel="noreferrer">ui-config documentation</a>
  ):</p>
  
  <pre><code>Accounts.ui.config({
    passwordSignupFields: 'USERNAME_AND_OPTIONAL_EMAIL'
  });
  
  
  Error: TypeError: Cannot call method 'config' of undefined
  </code></pre>