Include base version for msgpack, because 0.3 doesn't work

If I import neovim with less than version 0.4 of msgpack installed, I get this stack trace:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/chartdev/venvs/chartio/local/lib/python2.7/site-packages/neovim/__init__.py", line 8, in <module>
    from .api import DecodeHook, Nvim, SessionHook
  File "/home/chartdev/venvs/chartio/local/lib/python2.7/site-packages/neovim/api/__init__.py", line 9, in <module>
    from .nvim import Nvim, NvimError
  File "/home/chartdev/venvs/chartio/local/lib/python2.7/site-packages/neovim/api/nvim.py", line 4, in <module>
    from msgpack import ExtType
ImportError: cannot import name ExtType
```

0.4.0 seems to work.

Author: benekastah

setup.py

@@ -4,7 +4,7 @@
 from setuptools import setup
 
 install_requires = [
-    'msgpack-python',
+    'msgpack-python>=0.4.0',
 ]
 
 if sys.version_info < (3, 4):