Change to msgpack from msgpack-python (fluent#127)

* Update setup.py

Author: Kazuki Matsuda

setup.py

@@ -17,7 +17,7 @@
   long_description=open(README).read(),
   package_dir={'fluent': 'fluent'},
   packages=['fluent'],
-  install_requires=['msgpack-python'],
+  install_requires=['msgpack'],
   author='Kazuki Ohta',
   author_email='[email protected]',
   url='https://github.com/fluent/fluent-logger-python',