setup.py: Loosen the pyVows dependency to blacklist bad versions

1d264b9 (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 tried to fix this issue 59e67374 (topics
can be none, we need to validate against that, 2014-06-02, [1]) but
failed [2].  Instead of ignoring all future pyVows development, only
blacklist the versions we know to be broken.

[1]: heynemann/pyvows@59e6737
[2]: heynemann/pyvows#115
     Summary: runner.gevent: Shift None check for topic_func earlier
     Date: 2014-10-08

setup.py

@@ -54,7 +54,7 @@
         'test': tests_require,
     },
     install_requires=[
-        "pyvows<=2.0.3",
+        "pyvows !=2.0.4, !=2.0.5, !=2.0.6",
         "tornado",
         "pycurl",
         "urllib3"