BigQuery integration tests are skipped on travis
Certain BigQuery integration tests are being skipped because they require a project id. 

Valid BigQuery credentials are required to properly test the gbq module on travis.

See PR #10857 

```
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#18 pandas.io.tests.test_gbq.TestGBQConnectorIntegration.test_should_be_able_to_get_a_bigquery_service: Cannot run integration tests without a project id
#19 pandas.io.tests.test_gbq.TestGBQConnectorIntegration.test_should_be_able_to_get_results_from_query: Cannot run integration tests without a project id
#20 pandas.io.tests.test_gbq.TestGBQConnectorIntegration.test_should_be_able_to_get_schema_from_query: Cannot run integration tests without a project id
#21 pandas.io.tests.test_gbq.TestGBQConnectorIntegration.test_should_be_able_to_get_valid_credentials: Cannot run integration tests without a project id
#22 pandas.io.tests.test_gbq.TestGBQConnectorIntegration.test_should_be_able_to_make_a_connector: Cannot run integration tests without a project id
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#23 <nose.suite.ContextSuite context=TestReadGBQIntegration>:setup: Cannot run integration tests without a project id
#24 <nose.suite.ContextSuite context=TestToGBQIntegration>:setup: Cannot run integration tests without a project id
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```
