Keep IE from caching ajax call

gistfile1

function getCart(callback) {

  var rnum = Math.floor((Math.random()*1000000000)+1),
    rand_url = '/cart.js?' + rnum;

  jQuery.getJSON(rand_url, function (cart, textStatus) {
    if ((typeof callback) === 'function') {
      callback(cart);
    }
    else {
      Shopify.onCartUpdate(cart);
    }
  });
}