[doc] Documentation on readthedocs doesn't track stable release

[skomanduri]

Fantastic work so far!

The hosted documentation at http://docs.bonobo-project.org/en/master/reference/index.html only shows documentation for master and develop but the title of the page is "Reference — Bonobo 0.6.4 documentation" and this is in the top nav of the page also.

This was confusing to us as we found that a feature did not work as documented. We were able to fix it by installing the 0.7.0rc3 pre-release.

Expected: I should be able to read the documentation for the last stable release OR it should be clear that I am reading documentation for a dev version.

Thanks!