Update MathJax CDN, closes #881

liquid_tags/notebook.py

@@ -167,7 +167,7 @@ div.collapseheader {
 }
 </style>
 
-<script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML" type="text/javascript"></script>
+<script type="text/javascript" async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-AMS_HTML"></script>
 <script type="text/javascript">
 init_mathjax = function() {
     if (window.MathJax) {