Fixes Thinking Sphinx 2.x compatibility.

Thinking Sphinx 2.x uses `config/sphinx.yml` while 3.x usings
`config/thinking_sphinx.yml`. This change introduces the `:sphinx_yml`
setting which can be used to specify while configuration file to use
based on the version of Thinking Sphinx you are using.

Also fixes the download URL so that it works with the "archived" 0.9.8.1
release and newer 2.x releases of Sphinx.