Merge pull request #6395 from minrk/mathjax-url-nbconvert

update mathjax CDN url in nbconvert template

IPython/nbconvert/templates/html/mathjax.tpl

@@ -1,6 +1,6 b''
 {%- macro mathjax() -%}
     <!-- Load mathjax -->
-    <script src="https://c328740.ssl.cf1.rackcdn.com/mathjax/latest/MathJax.js?config=TeX-AMS_HTML"></script>
+    <script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML"></script>
     <!-- MathJax configuration -->
     <script type="text/x-mathjax-config">
     MathJax.Hub.Config({