Monitor hangs when API is unreachable

[mammuth]

Bitfinex API is unreachable currently.
Probably would be a smaller problem, if each update process would be async itself (see #23).

Traceback (most recent call last):
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/urllib3/connectionpool.py", line 380, in _make_request
    httplib_response = conn.getresponse(buffering=True)
TypeError: getresponse() got an unexpected keyword argument 'buffering'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib64/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib64/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/max/Code/bitcoin-arbitrage/bitcoin_arbitrage/__main__.py", line 9, in <module>
    monitor.start()  # ToDo: This call should not be blocking
  File "/home/max/Code/bitcoin-arbitrage/bitcoin_arbitrage/monitor.py", line 29, in start
    self._update_task_loop.run_until_complete(self._update_task)
  File "/usr/lib64/python3.6/asyncio/base_events.py", line 454, in run_until_complete
    self.run_forever()
  File "/usr/lib64/python3.6/asyncio/base_events.py", line 421, in run_forever
    self._run_once()
  File "/usr/lib64/python3.6/asyncio/base_events.py", line 1426, in _run_once
    handle._run()
  File "/usr/lib64/python3.6/asyncio/events.py", line 127, in _run
    self._callback(*self._args)
  File "/home/max/Code/bitcoin-arbitrage/bitcoin_arbitrage/monitor.py", line 42, in _update
    exchange.update_prices()
  File "/home/max/Code/bitcoin-arbitrage/bitcoin_arbitrage/exchange/bitfinex.py", line 20, in update_prices
    response = requests.get(url)
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/requests/api.py", line 72, in get
    return request('get', url, params=params, **kwargs)
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/requests/api.py", line 58, in request
    return session.request(method=method, url=url, **kwargs)
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/requests/sessions.py", line 508, in request
    resp = self.send(prep, **send_kwargs)
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/requests/sessions.py", line 618, in send
    r = adapter.send(request, **kwargs)
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/requests/adapters.py", line 440, in send
    timeout=timeout
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/urllib3/connectionpool.py", line 601, in urlopen
    chunked=chunked)
  File "/home/max/.virtualenvs/bitcoin-arbitrage-5sdT_1ZK/lib/python3.6/site-packages/urllib3/connectionpool.py", line 383, in _make_request
    httplib_response = conn.getresponse()
  File "/usr/lib64/python3.6/http/client.py", line 1331, in getresponse
    response.begin()
  File "/usr/lib64/python3.6/http/client.py", line 297, in begin
    version, status, reason = self._read_status()
  File "/usr/lib64/python3.6/http/client.py", line 258, in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
  File "/usr/lib64/python3.6/socket.py", line 586, in readinto
    return self._sock.recv_into(b)
  File "/usr/lib64/python3.6/ssl.py", line 1009, in recv_into
    return self.read(nbytes, buffer)
  File "/usr/lib64/python3.6/ssl.py", line 871, in read
    return self._sslobj.read(len, buffer)
  File "/usr/lib64/python3.6/ssl.py", line 631, in read
    v = self._sslobj.read(len, buffer)
KeyboardInterrupt