[SPARK-2252] Fix MathJax for HTTPs.

Found out about this from the Hacker News link to GraphX which was using HTTPs.

@mengxr

Author: Reynold Xin <rxin@apache.org>

Closes #1189 from rxin/mllib-doc and squashes the following commits:

5328be0 [Reynold Xin] [SPARK-2252] Fix MathJax for HTTPs.

(cherry picked from commit 420c1c3e1beea03453e0eb9dc06f226c80496d68)
Signed-off-by: Reynold Xin <rxin@apache.org>