require msgpack >= 0.4.6 and < 0.6.0, forbid 0.5.0.

maybe this is the easiest way for us to deal with msgpack compatibility.

0.5.0 release had some troubles:
- FutureWarning on stderr disturbing other output there, breaking tests
- pip install -U broken due to a pip issue with the transisition pkg
  which was needed due to the package rename (ImportError for msgpack)
- some linux dists not packaging the transition pkg

setup.py

@@ -37,8 +37,8 @@ on_rtd = os.environ.get('READTHEDOCS')
 
 install_requires = [
     # msgpack pure python data corruption was fixed in 0.4.6.
-    # Also, we might use some rather recent API features.
-    'msgpack-python>=0.4.6',
+    # msgpack 0.5.0 was a bit of a troublemaker.
+    'msgpack-python>=0.4.6,!=0.5.0,<0.6.0',
     'pyzmq',
 ]