I think this is the correct way to do this now:

  <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
  <script>
    (function(){ if(!window.jQuery) {
      var s = document.createElement("script"),
          s0 = document.getElementsByTagName('script')[0];
      s.src = "../lib/js/jquery-2.1.1.min.js";
      s.async = false;
      s0.parentNode.insertBefore(s, s0);
    }})();
  </script>

EDIT:

Saving a few bytes although a little less readable:

  <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
  <script>(function(w,d,x,t,p,s,z){if(!w[t]){s=d.createElement(x);s.src=p;
  s.async=false;z=d.getElementsByTagName(x)[0];z.parentNode.insertBefore(s,z);}
  })(window,document,'script','jQuery','../lib/js/jquery-2.1.1.min.js');</script>