Bootstrap bottom align column within row

by RationalGeek

HTML

<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.0/css/bootstrap.min.css">
<script src="//netdna.bootstrapcdn.com/bootstrap/3.1.0/js/bootstrap.min.js"></script>
<div class="row">
    <div class="col-xs-6 mainBox">
        <h1>Heading</h1>
        <h2>Sub Heading</h2>
    </div>
    <div class="col-xs-6 mainBox buttonBox">
        <button class="btn btn-primary">Button</button>
    </div>
</div>

CSS

.mainBox {
    border: solid thin black;
}

.buttonBox {
    /* What goes here?!? */
}