fix ssl_version exception when urllib3 version <= 1.5

[li45]

on centos6 ,default urllib3 version is 1.5 .
HTTPSConnectionPool have not param ssl_version
got exception:

>>> client = docker.Client(base_url='https://127.0.0.1:2735', tls=tls_config)
>>> client.containers()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/site-packages/docker/client.py", line 445, in containers
    res = self._result(self._get(u, params=params), True)
  File "/usr/lib/python2.6/site-packages/docker/client.py", line 82, in _get
    return self.get(url, **self._set_request_timeout(kwargs))
  File "/usr/lib/python2.6/site-packages/requests/sessions.py", line 310, in get
    return self.request('GET', url, **kwargs)
  File "/usr/lib/python2.6/site-packages/requests/sessions.py", line 279, in request
    resp = self.send(prep, stream=stream, timeout=timeout, verify=verify, cert=cert, proxies=proxies)
  File "/usr/lib/python2.6/site-packages/requests/sessions.py", line 374, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python2.6/site-packages/requests/adapters.py", line 155, in send
    conn = self.get_connection(request.url, proxies)
  File "/usr/lib/python2.6/site-packages/requests/adapters.py", line 125, in get_connection
    conn = self.poolmanager.connection_from_url(url)
  File "/usr/lib/python2.6/site-packages/urllib3/poolmanager.py", line 100, in connection_from_url
    return self.connection_from_host(u.host, port=u.port, scheme=u.scheme)
  File "/usr/lib/python2.6/site-packages/urllib3/poolmanager.py", line 84, in connection_from_host
    pool = pool_cls(host, port, **self.connection_pool_kw)
TypeError: __init__() got an unexpected keyword argument 'ssl_version'

[shin-]

Thank you for the contribution! Please make sure the tests pass :)

[shin-]

Thanks!