Change MathJax URL from deprecated Rackspace CDN

Use new cdn.mathjax.org URL

More information available here:
http://www.mathjax.org/changes-to-the-mathjax-cdn/

liquid_tags/notebook.py

@@ -141,7 +141,7 @@ div.collapseheader {
 }
 </style>
 
-<script src="https://c328740.ssl.cf1.rackcdn.com/mathjax/latest/MathJax.js?config=TeX-AMS_HTML" type="text/javascript"></script>
+<script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML" type="text/javascript"></script>
 <script type="text/javascript">
 init_mathjax = function() {
     if (window.MathJax) {