pypi: correct msgpack library name

In short, the msgpack library changes its name and now offered as
'msgpack' rather than 'msgpack-python'. All new versions of the library
are shipped under the new name, so we should change our dependency
information.

See commit 148d12e ('travis-ci: verify
different msgpack library versions') for details.

Follows up #155

Author: Totktonada

setup.py

@@ -83,6 +83,6 @@ def find_version(*file_paths):
     cmdclass=cmdclass,
     command_options=command_options,
     install_requires=[
-        'msgpack-python>=0.4',
+        'msgpack>=0.4.0',
     ]
 )