prevent moving links out of their drag containers

Without this fix, if you had dom like this:

```html
<div class="sortable">
  <div><a href="#">foo</a></div>
  <div><a href="#">bar</a></div>
</div>
```

and then dragged the first link below the second one, you had dom like this:

```html
<div class="sortable">
  <div></div>
  <div><a href="#">bar</a></div>
  <a href="#">foo</a>
</div>
```

This makes sure that you're always dealing with the most top level possible element.