SignedJwtAssertionCredentials has been removed from oauth2client version 2.00 breaking read_gbq
In pandas/io/gbq.py there is the function:

`def _test_google_api_imports():

```
try:
    import httplib2  # noqa
    from apiclient.discovery import build  # noqa
    from apiclient.errors import HttpError  # noqa
    from oauth2client.client import AccessTokenRefreshError  # noqa
    from oauth2client.client import OAuth2WebServerFlow  # noqa
    from oauth2client.client import SignedJwtAssertionCredentials  # noqa
    from oauth2client.file import Storage  # noqa
    from oauth2client.tools import run_flow, argparser  # noqa
except ImportError as e:
    raise ImportError("Missing module required for Google BigQuery "
                      "support: {0}".format(str(e)))`
```

This will fail for users with oauth2client version higher than 2.0.0 since SignedJwtAssertionCredentials  has been removed from the module. 

Google suggests other ways of handling service account credentials. See:
https://github.com/google/oauth2client/blob/master/CHANGELOG.md#v200
