return preventDefault calls to touch event

refs #130
- some browsers don't support touch-action: none, so we need to call
event.preventDefault anways to make sure it covers all browsers

addon/mixins/sortable-item.js

@@ -307,6 +307,9 @@ export default Mixin.create({
       return;
     }
 
+    event.preventDefault();
+    event.stopPropagation();
+
     this._startDragListener = event => this._startDrag(event);
 
     this._cancelStartDragListener = () => {