Fix deprecation warnings for msgpack > 0.5.2

[angeloudy]

I am getting the following warnings

09:58:35,223 [WARNING ] /usr/local/lib/python3.6/site-packages/salt/transport/ipc.py:280: DeprecationWarning: encoding is deprecated, Use raw=False instead.
  self.unpacker = msgpack.Unpacker(encoding=encoding)

This commit fixed the problem

[waynew]

We have an existing PR that will fix the problem across the board. However, every single message from Salt master <-> minion gets piped through this transport layer, and while we have enough tests to give me a 90% confidence level that this won't break everything...

Given the potential risk, I would like to get more complete tests in place ensuring that these changes will do nothing wrong when it comes to Salt's messages.

Unfortunately, I've got some other, higher-priority items on my plate currently so I haven't been able to devote the time I would like to getting that fixed. If you're interested, more complete transport level tests would be most welcome!

[waynew]

@angeloudy I'm going to go ahead and close this PR, since it's pointing to develop, and @Akm0d I know made some changes to this recently on master. If you have any questions, please feel free to reach out!