Read the Docs is now using the latest version of Sphinx, so upgrade our
recommended version too.
As of Sphinx 1.4, index entries are 5-tuples instead of 4-tuples. Update
the Cretonne Sphinx domain to generate the new 5-tuples.
Since we're over this compatibility bump, there's no reason to recommend
a specific Sphinx version, so just go back to 'current'.