Update jQuery to a more recent version in the docs

I [note](https://github.com/harvesthq/chosen/blob/master/public/index.html#L1461) it's at 1.6.4 while jQuery is now at 1.12.1. It's suggested that people use a recent build of both chosen and jQuery, the docs should probably be doing the same!

``` html
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js" type="text/javascript"></script>
```
