Update use_connection_pooling (False) to address session overwriting

[rollerbrick]

When using CustomerIO with use_connection_pooling set to False the following error is occasionally produced. This PR changes use_connection_pooling (False) so it doesn't use self._current_session.

Traceback (most recent call last):
  ...
  File "...customerio/track.py", line 72, in identify
    self.send_request('PUT', url, kwargs)
  File "...customerio/client_base.py", line 55, in send_request
    self._close()
  File "...customerio/client_base.py", line 112, in _close
    self._current_session.close()
AttributeError: 'NoneType' object has no attribute 'close'

[hownowstephen]

Superseded by #118 (merged), which fixed no-pooling session isolation.