Remove deprecated usage of msgpack (https://github.com/tensorflow/tensorflow/issues/16906)

requirements.txt

@@ -3,7 +3,7 @@ six
 termcolor>=1.1
 tabulate>=0.7.7
 tqdm>4.11.1
-msgpack
+msgpack>=0.5.2
 msgpack-numpy>=0.4.0
 pyzmq>=16
 subprocess32; python_version < '3.0'

tensorpack/utils/serialize.py

@@ -34,7 +34,7 @@ def loads_msgpack(buf):
     Args:
         buf: the output of `dumps`.
     """
-    return msgpack.loads(buf, encoding='utf-8')
+    return msgpack.loads(buf, raw=False)
 
 
 def dumps_pyarrow(obj):