2.x not compatible with @html-next/vertical-collection

[BryanCrotaz]

Vertical collection expects the items provided by group.model to be an array or an object that implements objectAt and will assert if they're not.

Works in ember-sortable 1.x.

<SortableGroup model=myModel as |group|>
  {{#vertical-collection items=group.model as |item|}} 
    <SortableItem model={{item}}>
      <div>{{item.name}}</div>
    </SortableItem>
  {{/vertical-collection}}
</SortableGroup>