title: Braintree - paymentMethodNonceReceived not being invoked
body: <p>I have setup dropin UI for braintree. I can see the UI fine. Before that I created the customer and I can see the customer on braintree-sandbox. Now I want to add payment method to the customer. I am trying following code, but paymentMethodNonceReceived is not being invoked. Not sure why.</p>
  
  <pre><code>braintree.setup("&lt;?=CLIENT_TOKEN_FROM_PHP?&gt;", 
      "dropin", 
      {
        container: "divBrainTreeContainer",
        paymentMethodNonceReceived: function (event, nonce) {
          console.log(nonce);
          $('#formProfile').append('&lt;input type="hidden" name="payment_method_nonce" value="'+nonce+'" /&gt;');
          $('#formProfile').submit();
        }
      }
  );
  </code></pre>