Only expand log directory path if it exists.

It'd be nice to rely on the log directory always existing, but sadly this is not the case on Heroku (at least, not consistently). So, if it is there, the expanded path (avoiding linked directories) will be used, but otherwise, we don't really care.
Default Sphinx version is 2.1.4.

This is (as far as I know) only used by Thinking Sphinx. Let's do our best to keep people on modern versions of Sphinx as much as possible.