nvim.async_call() + asyncio is broken

[Shougo]

Please see the issues.

tweekmonster/deoplete-clang2#30
wsdjeg/SpaceVim#1588

I have get the error reason is nvim.async_call() + asyncio.

@bfredl Can you test it?

python-client error messages are here:

2018-04-14 19:44:01,167 [ERROR @ base_events.py:default_exception_handler:1265] 29966 - Task exception was never retrieved
future: <Task finished coro=<BaseEventLoop.subprocess_exec() done, defined at /usr/lib/python3.6/asyncio/base_events.py:1166> exception=RuntimeError('Cannot add child handler, the child watcher does not have a loop attached',)>
Traceback (most recent call last):
  File "/usr/lib/python3.6/asyncio/base_events.py", line 1191, in subprocess_exec
    bufsize, **kwargs)
  File "/usr/lib/python3.6/asyncio/unix_events.py", line 194, in _make_subprocess_transport
    self._child_watcher_callback, transp)
  File "/usr/lib/python3.6/asyncio/unix_events.py", line 858, in add_child_handler
    "Cannot add child handler, "
RuntimeError: Cannot add child handler, the child watcher does not have a loop attached