Update collapse.js

What is the purpose of using the ".panel" selector? It doesn't work for 

```html
...
<li><a class="accordion-toggle collapsed" data-toggle="collapse" data-parent="#mainmenu" href="#collapseOne">label</a>
<ul class="list-unstyled collapse" id="collapseOne">
<li><a href="#">label</a></li>
</ul>
</li>
...
```