Bootstrap modal is not a function

Question

When I call my model in windows it gives an error to the console.
This is the error:

$(...).modal is not a function

This is my HTML :

<div class="modal fade" id="prizePopup" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
    <div class="modal-content">
        <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">
                &times;
            </button>
            <h4 class="modal-title" id="myModalLabel">
                This Modal title
            </h4>
        </div>
        <div class="modal-body">
            Add some text here
        </div>
        <div class="modal-footer">
            <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
            <button type="button" class="btn btn-primary">
                Submit changes
            </button>
        </div>
    </div>
</div>
<script type="text/javascript" src="js/bootstrap.js"></script>
<script src="//code.jquery.com/jquery-1.11.0.min.js"></script>

And this is my JavaScript :

<script type="text/javascript">
$(window).load(function() {
    $('#prizePopup').modal('show');
});
</script>

How can I solve this error?