Install requires msgpack

Fresh from install on Debian 7:
```pyhton
>>> from raft import server
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/raft/server.py", line 9, in <module>
    import msgpack
ImportError: No module named msgpack
>>>
```