Fix msgpack PendingDeprecationWarning for encoding='utf-8'

[ogrisel]

This should get rid of deprecation warnings such as:

/home/travis/miniconda/envs/testenv/lib/python3.6/site-packages/distributed/protocol/core.py:188: PendingDeprecationWarning: encoding is deprecated, Use raw=False instead.
  return msgpack.loads(payload, encoding='utf8', use_list=False)

while preserving backward compat for older versions of msgpack. Unfortunately it's not possible introspect the version number of the msgpack package, hence the TypeError except block.

[ogrisel]

@mrocklin the appveyor job failed with:

Traceback (most recent call last):
  File "C:\projects\distributed\distributed\tests\test_client.py", line 5440, in test_no_threads_lingering
    assert threading.active_count() < 30, list(active.values())
AssertionError: [<Thread(AsyncProcess Process-74 watch message queue, started daemon 2560)>, <Thread(AsyncProcess Process-350 watch pr...eaded scatter(), started daemon 680)>, <Thread(AsyncProcess Process-11 watch message queue, started daemon 2252)>, ...]
assert 30 < 30
 +  where 30 = <function active_count at 0x000000490A6FF8C8>()
 +    where <function active_count at 0x000000490A6FF8C8> = threading.active_count

I don't think this is caused by this PR.

For reference, I have run the distributed/protocol/tests/ locally with msgpack==0.5.1 and they pass.

[mrocklin]

This looks fine to me. Sorry about the intermittent failures. This one is actually caused by a recent PR to clean up other intermittent failures:

• Cleanup #2146
• Cleanup recent cleanup PR #2152

I'll take care of it.

[ogrisel]

Alright merged!

[mrocklin]

Thanks @ogrisel