Add support for index entry format of Sphinx v1.4
Add support for index entry format of Sphinx v1.4

Sphinx added a fifth element to index entries (see commit
sphinx-doc/sphinx@e6a5a3a92e938fcd75866b4227db9e0524d58f7c).
This change adds support for that fifth element if running
Sphinx v1.4.0 or later.

Related to #3.

t4ngo/sphinxcontrib-traceables