Replace oauth2client by google-auth

[ktong]

oauth2client is now deprecated. No more features will be added to the libraries and the core team is turning down support. We recommend you use google-auth and oauthlib.

[tswast]

The latest versions of google-api-python-client support google-auth. See: googleapis/google-api-python-client#397 (comment) This package should be able to do this with only some changes to setup.py.

[ktong]

It also need change the way to get application default credentials

try:
            from oauth2client.client import GoogleCredentials
        except ImportError:
            return None

        try:
            credentials = GoogleCredentials.get_application_default()
        except:
            return None

[ktong]

the new function is google.auth.default()

[tswast]

I started on this in https://github.com/tswast/pandas-gbq/tree/gbq-37. It uses google-auth for service accounts and application default credentials. I'll update it to use google-auth for user credentials tomorrow.