Backport PR #6249: always use HTTPS getting mathjax from CDN

Is there any reason not to do this now?

closes  #6246

IPython/html/notebookapp.py

@@ -516,7 +516,7 b' class NotebookApp(BaseIPythonApplication):'
                 return url
         
         # no local mathjax, serve from CDN
-        url = u"//cdn.mathjax.org/mathjax/latest/MathJax.js"
+        url = u"https://cdn.mathjax.org/mathjax/latest/MathJax.js"
         self.log.info("Using MathJax from CDN: %s", url)
         return url