setup.py: Loosen the pyVows dependency to blacklist only 2.0.4 and 2.0.5

1d264b96 (Pyvows 2.0.5 has broken tornado_pyvows, 2014-05-19)
mentioned pyVows 2.0.5, but also blocked 2.0.4.  Since then, there has
been a 2.0.6 release which fixes this issue 59e67374 (topics can be
none, we need to validate against that, 2014-06-02, [1]).  Instead of
ignoring all future pyVows development, only blacklist the versions we
know to be broken.  Well, I haven't actually tested with 2.0.4, but we
should be allowing 2.0.6 (which I have tested) and later.

[1]: https://github.com/heynemann/pyvows/commit/59e67374bcfbf7c8c45a4be575e8eae64902c209