Newer new_widget API takes 3rd param.

Ref jupyter#141
Ref jupyter#196

(c) Copyright IBM Corp. 2016

elements/urth-core-behaviors/jupyter-widget-behavior.html

@@ -49,7 +49,8 @@
                     {
                         model_name: 'DeclWidgetModel',
                         widget_class: this.kernelClass
-                    }
+                    },
+                    {}
                 ).then(function(model) {
                         console.log('Model creation successful!', model);
                         this.__modelChangeCallback = this._onModelChange.bind(this);