Fix failing tests

[kumaresan-plivo]

1. Fix failing travis CI builds due to --use-mirrors
2. Tests currently need real Auth credentials to work. Come up with infrastructure to do this or use mock tests.

[wojcikstefan]

We've been successfully testing our requests to Plivo and mocking Plivo responses with https://github.com/patrys/httmock. A sample code looks like this:

def test_delete_phone_number(self):
    requests_list = []  # track all the requests made to plivo

    @urlmatch(netloc='api\.plivo\.com')
    def response_content(url, request):
        requests_list.append((request.method, url.geturl()))
        return { 'status_code': 204, 'content': '' }

    with HTTMock(response_content):
        resp = self.delete('/api/v1/phone_number/{}/'.format(pn.pk), api_key=test_api_key)
        response_success(resp)

    # Assert that we only made a single DELETE request to Plivo
    self.assertEqual(requests_list, [
        ('DELETE', 'https://api.plivo.com/v1/Account/TEST_PLIVO_AUTH_ID/Number/16503334444/')
    ])

You can also have a more sophisticated logic of what to respond with depending on the request data/url and you can put asserts inside of response_content. Hope this is helpful.

[kumaresan-plivo]

Thanks @wojcikstefan, sounds like a good fit.