Fix issue using Collapse without Panel

Overview
---
If you try to use Collapse without Panel, you **must** use the `.panel` class to get it work.

Example
---
```
<div id="accordion">
  <div>
    <a data-toggle="collapse" data-parent="#accordion" href="#collapseOne">Item #1</a>
    <div id="collapseOne" class="collapse in">
      Lorem Ipsum
    </div>
  </div>
  <div>
    <a data-toggle="collapse" data-parent="#accordion" href="#collapseTwo">Item #2</a>
    <div id="collapseTwo" class="collapse">
      Lorem Ipsum
    </div>
  </div>
  <div>
    <a data-toggle="collapse" data-parent="#accordion" href="#collapseThree">Item #3</a>
    <div id="collapseThree" class="collapse">
      Lorem Ipsum
    </div>
  </div>
</div>
```

Expected behavior
---
On `Item #2` click, we expect the first block (under `Item #1`) to collapse, and the second one (under `Item #2`) to extend.

Current behavior
---
On `Item #2` click, and the second block (under `Item #2`) extends. The first block (under `Item #1`) doesn't collapse.