remove ie8 support

@t4t5 Not sure what browsers you want to support.

With this pull if someone wanted IE8 support they would need something like this in the page


```html
<!--[if IE 8]>
    <script src="//cdnjs.cloudflare.com/ajax/libs/es5-shim/4.0.3/es5-shim.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/classlist/2014.01.31/classList.min.js"></script>
<![endif]-->
....
<script src="lib/sweet-alert.min.js"></script>
...
```