CI: GBQ tests failing on master
https://travis-ci.org/pandas-dev/pandas/jobs/569827581

```
_________ TestToGBQIntegrationWithServiceAccountKeyPath.test_roundtrip _________
2328[gw0] linux -- Python 3.6.6 /home/travis/miniconda3/envs/pandas-dev/bin/python
2329
2330self = <pandas.tests.io.test_gbq.TestToGBQIntegrationWithServiceAccountKeyPath object at 0x7fb10497cbe0>
2331
2332    def test_roundtrip(self):
2333        destination_table = DESTINATION_TABLE + "1"
2334    
2335        test_size = 20001
2336        df = make_mixed_dataframe_v2(test_size)
2337    
2338        df.to_gbq(
2339            destination_table,
2340            _get_project_id(),
2341            chunksize=None,
2342            credentials=_get_credentials(),
2343        )
2344    
2345        result = pd.read_gbq(
2346            "SELECT COUNT(*) AS num_rows FROM {0}".format(destination_table),
2347            project_id=_get_project_id(),
2348            credentials=_get_credentials(),
2349            dialect="standard",
2350        )
2351>       assert result["num_rows"][0] == test_size
2352E       assert 0 == 20001
2353
2354pandas/tests/io/test_gbq.py:194: AssertionError
2355- generated xml file: /home/travis/build/pandas-dev/pandas/test-data-single.xml -
2356
```

https://travis-ci.org/pandas-dev/pandas/jobs/569818532

```
==================================== ERRORS ====================================
2907_ ERROR at teardown of TestToGBQIntegrationWithServiceAccountKeyPath.test_roundtrip _
2908[gw0] linux -- Python 3.6.9 /home/travis/miniconda3/envs/pandas-dev/bin/python
2909
2910cls = <class 'pandas.tests.io.test_gbq.TestToGBQIntegrationWithServiceAccountKeyPath'>
2911
2912    @classmethod
2913    def teardown_class(cls):
2914        # - GLOBAL CLASS FIXTURES -
2915        # put here any instruction you want to execute only *ONCE* *AFTER*
2916        # executing all tests.
2917>       cls.client.delete_dataset(cls.dataset, delete_contents=True)
2918
2919pandas/tests/io/test_gbq.py:173: 
2920_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
2921../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/google/cloud/bigquery/client.py:627: in delete_dataset
2922    retry, method="DELETE", path=dataset.path, query_params=params
2923../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/google/cloud/bigquery/client.py:400: in _call_api
2924    return call()
2925../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/google/api_core/retry.py:273: in retry_wrapped_func
2926    on_error=on_error,
2927../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/google/api_core/retry.py:182: in retry_target
2928    return target()
2929_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
2930
2931self = <google.cloud.bigquery._http.Connection object at 0x7f0c41a5a160>
2932method = 'DELETE'
2933path = '/projects/pandas-travis/datasets/pydata_pandas_bq_testing_py31'
2934query_params = {'deleteContents': 'true'}, data = None, content_type = None
2935headers = None, api_base_url = None, api_version = None, expect_json = True
2936_target_object = None
2937
2938    def api_request(
2939        self,
2940        method,
2941        path,
2942        query_params=None,
2943        data=None,
2944        content_type=None,
2945        headers=None,
2946        api_base_url=None,
2947        api_version=None,
2948        expect_json=True,
2949        _target_object=None,
2950    ):
2951        """Make a request over the HTTP transport to the API.
2952    
2953        You shouldn't need to use this method, but if you plan to
2954        interact with the API using these primitives, this is the
2955        correct one to use.
2956    
2957        :type method: str
2958        :param method: The HTTP method name (ie, ``GET``, ``POST``, etc).
2959                       Required.
2960    
2961        :type path: str
2962        :param path: The path to the resource (ie, ``'/b/bucket-name'``).
2963                     Required.
2964    
2965        :type query_params: dict or list
2966        :param query_params: A dictionary of keys and values (or list of
2967                             key-value pairs) to insert into the query
2968                             string of the URL.
2969    
2970        :type data: str
2971        :param data: The data to send as the body of the request. Default is
2972                     the empty string.
2973    
2974        :type content_type: str
2975        :param content_type: The proper MIME type of the data provided. Default
2976                             is None.
2977    
2978        :type headers: dict
2979        :param headers: extra HTTP headers to be sent with the request.
2980    
2981        :type api_base_url: str
2982        :param api_base_url: The base URL for the API endpoint.
2983                             Typically you won't have to provide this.
2984                             Default is the standard API base URL.
2985    
2986        :type api_version: str
2987        :param api_version: The version of the API to call.  Typically
2988                            you shouldn't provide this and instead use
2989                            the default for the library.  Default is the
2990                            latest API version supported by
2991                            google-cloud-python.
2992    
2993        :type expect_json: bool
2994        :param expect_json: If True, this method will try to parse the
2995                            response as JSON and raise an exception if
2996                            that cannot be done.  Default is True.
2997    
2998        :type _target_object: :class:`object`
2999        :param _target_object:
3000            (Optional) Protected argument to be used by library callers. This
3001            can allow custom behavior, for example, to defer an HTTP request
3002            and complete initialization of the object at a later time.
3003    
3004        :raises ~google.cloud.exceptions.GoogleCloudError: if the response code
3005            is not 200 OK.
3006        :raises ValueError: if the response content type is not JSON.
3007        :rtype: dict or str
3008        :returns: The API response payload, either as a raw string or
3009                  a dictionary if the response is valid JSON.
3010        """
3011        url = self.build_api_url(
3012            path=path,
3013            query_params=query_params,
3014            api_base_url=api_base_url,
3015            api_version=api_version,
3016        )
3017    
3018        # Making the executive decision that any dictionary
3019        # data will be sent properly as JSON.
3020        if data and isinstance(data, dict):
3021            data = json.dumps(data)
3022            content_type = "application/json"
3023    
3024        response = self._make_request(
3025            method=method,
3026            url=url,
3027            data=data,
3028            content_type=content_type,
3029            headers=headers,
3030            target_object=_target_object,
3031        )
3032    
3033        if not 200 <= response.status_code < 300:
3034>           raise exceptions.from_http_response(response)
3035E           google.api_core.exceptions.NotFound: 404 DELETE https://www.googleapis.com/bigquery/v2/projects/pandas-travis/datasets/pydata_pandas_bq_testing_py31?deleteContents=true: Not found: Dataset pandas-travis:pydata_pandas_bq_testing_py31
3036
3037../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/google/cloud/_http.py:319: NotFound
3038=================================== FAILURES ===================================
3039_________ TestToGBQIntegrationWithServiceAccountKeyPath.test_roundtrip _________
3040[gw0] linux -- Python 3.6.9 /home/travis/miniconda3/envs/pandas-dev/bin/python
3041
3042self = <pandas.tests.io.test_gbq.TestToGBQIntegrationWithServiceAccountKeyPath object at 0x7f0c41a5a400>
3043
3044    def test_roundtrip(self):
3045        destination_table = DESTINATION_TABLE + "1"
3046    
3047        test_size = 20001
3048        df = make_mixed_dataframe_v2(test_size)
3049    
3050        df.to_gbq(
3051            destination_table,
3052            _get_project_id(),
3053            chunksize=None,
3054>           credentials=_get_credentials(),
3055        )
3056
3057pandas/tests/io/test_gbq.py:185: 
3058_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
3059pandas/core/frame.py:1493: in to_gbq
3060    private_key=private_key,
3061pandas/io/gbq.py:209: in to_gbq
3062    private_key=private_key,
3063../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas_gbq/gbq.py:1204: in to_gbq
3064    progress_bar=progress_bar,
3065../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas_gbq/gbq.py:581: in load_data
3066    self.process_http_error(ex)
3067_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
3068
3069ex = NotFound('Not found: Dataset pandas-travis:pydata_pandas_bq_testing_py31',)
3070
3071    @staticmethod
3072    def process_http_error(ex):
3073        # See `BigQuery Troubleshooting Errors
3074        # <https://cloud.google.com/bigquery/troubleshooting-errors>`__
3075    
3076>       raise GenericGBQException("Reason: {0}".format(ex))
3077E       pandas_gbq.gbq.GenericGBQException: Reason: 404 Not found: Dataset pandas-travis:pydata_pandas_bq_testing_py31
3078
3079../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas_gbq/gbq.py:437: GenericGBQException
3080=============================== warnings summary ===============================
```