Update for sphinx internal structure change layout

As reported upstream in sphinx:
https://github.com/sphinx-doc/sphinx/pull/2396

It was a structural change in sphinx-1.4. I'm not sure if this is the
proper fix, but it does let the doc build pass, in my testing.

Originally reported at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=830178.

Signed-off-by: Nishanth Aravamudan <nish.aravamudan@canonical.com>