TST: update gbq service account key
Re-enable gbq integration tests.

- [ ] closes #34779
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Build currently running in my personal Travis config at https://travis-ci.org/github/tswast/pandas/builds/704081779

I need additional permissions in order to create the encryption key environment variables on the pandas-dev Travis config. @tswast does this run? @jreback It [runs on my fork](https://travis-ci.org/github/tswast/pandas/builds/704081779), but still having trouble with encrypting the keys for use in the pandas-dev org.

```
$ travis encrypt-file -r pandas-dev/pandas $GBQ_JSON_FILE travis_gbq.json.enc -f

encrypting pandas-gbq-tests-e842e94bcd25.json for pandas-dev/pandas
storing result as travis_gbq.json.enc
storing secure env variables for decryption
resource not found ({"error":"Couldn't find repository"})
```

I can now see the pandas project under "my repositories" in Travis. But maybe it's still not working because I can't see the "Settings" menu like I can on my fork.

<img width="834" alt="Screen Shot 2020-07-06 at 9 45 32 AM" src="https://user-images.githubusercontent.com/247555/86606422-9461cf80-bf6d-11ea-85e8-1c51fd0f66a4.png">

<img width="836" alt="Screen Shot 2020-07-06 at 9 47 25 AM" src="https://user-images.githubusercontent.com/247555/86606564-c6733180-bf6d-11ea-9076-906a8a5399e8.png">

 Looks like write access is needed in order to update settings (including adding encryption keys?). 😕 https://docs.travis-ci.com/user/travis-ci-for-private/#who-has-access-to-the-builds I was able to add the required keys to Travis. Since Travis doesn't allow the use of keys from forks, I also pushed this branch https://github.com/pandas-dev/pandas/tree/issue34779-enable-gbq-tests, which can be deleted once verifying the build at https://travis-ci.org/github/pandas-dev/pandas/builds/705824144 ok let's try this.