Added braintree.js onload callback to prevent braintree not defined error (spree-contrib#254)

Author: mdavo6

app/views/spree/braintree_vzero/_paypal_checkout.html.erb

@@ -4,7 +4,7 @@
     <img src="https://www.paypalobjects.com/en_US/i/btn/btn_xpressCheckout.gif" id="btnOpenFlow">
   </div>
 
-  <script src="https://js.braintreegateway.com/v2/braintree.js"></script>
+  <script src="https://js.braintreegateway.com/v2/braintree.js" onload="braintreeLoaded()"></script>
   <script src="https://js.braintreegateway.com/js/braintree-2.32.1.min.js"></script>
 
   <script type="text/javascript">
@@ -84,10 +84,12 @@
       document.querySelector('#btnOpenFlow').addEventListener('click', function () { checkout.paypal.initAuthFlow(); }, false);
     }
 
-    if(document.readyState !== 'loading') {
-      initBraintreePayPal()
-    } else {
-      window.addEventListener('DOMContentLoaded', initBraintreePayPal);
+    function braintreeLoaded () {
+      if(document.readyState !== 'loading') {
+        initBraintreePayPal()
+      } else {
+        window.addEventListener('DOMContentLoaded', initBraintreePayPal);
+      }
     }
 
     function initBraintreePayPal() {