set use_ssl to True in the default configuration

[edenhaus]

Set use_ssl to True. Fixes #303

2 Unit tests are failing, but this tests are failing also, if we leave the option on False.

Failed unit tests

======================================================================
FAIL: test_get_coi (tests.unit.airpollutionapi30.test_airpollution_client.TestAirPollutionHttpClient)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/robert/Documents/github/pyowm/tests/unit/airpollutionapi30/test_airpollution_client.py", line 42, in test_get_coi
    self.assertEqual(expected_url, result)
AssertionError: 'co/43.75,8.25/current.json' != {'time': '2016-10-01T13:07:01Z', 'locatio[342 chars]07}]}

======================================================================
FAIL: test_get_o3 (tests.unit.airpollutionapi30.test_airpollution_client.TestAirPollutionHttpClient)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/robert/Documents/github/pyowm/tests/unit/airpollutionapi30/test_airpollution_client.py", line 93, in test_get_o3
    self.assertEqual(expected_url, result)
AssertionError: 'o3/43.75,8.25/current.json' != {'time': '2016-10-06T13:32:53Z', 'locatio[68 chars]0781}

I only have the free subscription, so please can you run the integration tests again.

[csparpa]

Thanks
I've also got a free subscription only
Will look into it