TST: Improve unit tests for GoogleCredentials.get_application_default()

[parthea]

The test test_get_application_default_credentials_returns_credentials() : https://github.com/pydata/pandas-gbq/blob/master/pandas_gbq/tests/test_gbq.py#L297 is skipped if _check_if_can_get_correct_default_credentials() returns false. This happens when default credentials are not available. In order to improve test coverage, specifically on Travis, we could (temporarily) populate the default credentials during testing in order to exercise the code that uses default credentials. We could unset the default credentials after testing.