ensire u2f object is accessible in a commonJS environment

vendor/assets/javascripts/u2f.js

@@ -745,4 +745,6 @@ u2f.getApiVersion = function(callback, opt_timeoutSeconds) {
         };
         port.postMessage(req);
     });
-};
\ No newline at end of file
+};
+
+window.u2f || (window.u2f = u2f);