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'.
Rust 1.12 did work at one point, but Travis is testing against the
current stable and beta releases, so that is the only versions we can
claim to support.
Fixes#51.
The latest Sphinx 1.4.4 produces lots of warnings about four-column indices.
We'll wait for Read the Docs to upgrade their systems before moving to the newer
Sphinx version.
