Why does my jQuery plugin show up as text in WordPress?

You can use this PHP code to output your slider’s initiator in your themes footer:

add_action('wp_footer','initiate_unslider');
function initiate_unslider () {
    ?>
    <script src="https://code.jquery.com/jquery-2.1.4.min.js"></script>
    <script src="/path/to/unslider.js"></script>
    <script>
        $(document).ready(function() { 
            $('.banner').unslider();
        });
    </script>
    <?php
}

Also it’s safer to add a $(document).ready before initiating your slider, in case some necessary components of your page has not been loaded yet.