The self.indexnode's tuple has changed in sphinx version 1.4, from a
former 4 element tuple to a 5 element tuple.

https://github.com/sphinx-doc/sphinx/commit/e6a5a3a92e938fcd75866b4227db9e0524d58f7cSigned-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>