Change msgpack-python requirement to msgpack
Change msgpack-python requirement to msgpack

This package has been renamed to `msgpack` starting with their `0.5.0` version. The old package seems likely to be deprecated soon, but this change will keep the dependency being updated as intended.

https://pypi.python.org/pypi/msgpack
https://pypi.python.org/pypi/msgpack-python
spikeekips/serf-python