TST: re-enable gbq tests
Fix clean-up steps to use the google.cloud.bigquery client
library directly, since some of the _Dataset and _Table
logic was removed from pandas-gbq.

Follow-up to #23281

- [x] tests added / passed

```
$ pytest pandas/tests/io/test_gbq.py
====== test session starts ======
platform darwin -- Python 3.6.4, pytest-3.6.2, py-1.5.3, pluggy-0.6.0
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/Users/swast/src/pandas/pandas/.hypothesis/examples')
rootdir: /Users/swast/src/pandas/pandas, inifile: setup.cfg
plugins: xdist-1.22.2, forked-0.2, cov-2.5.1, hypothesis-3.70.3
collected 2 items

pandas/tests/io/test_gbq.py ..           [100%]

====== slowest 10 test durations ======
12.55s call     pandas/tests/io/test_gbq.py::TestToGBQIntegrationWithServiceAccountKeyPath::test_roundtrip
1.30s setup    pandas/tests/io/test_gbq.py::TestToGBQIntegrationWithServiceAccountKeyPath::test_roundtrip
0.66s teardown pandas/tests/io/test_gbq.py::TestToGBQIntegrationWithServiceAccountKeyPath::test_roundtrip
0.18s call     pandas/tests/io/test_gbq.py::test_read_gbq_without_dialect_warns_future_change
0.00s setup    pandas/tests/io/test_gbq.py::test_read_gbq_without_dialect_warns_future_change
0.00s teardown pandas/tests/io/test_gbq.py::test_read_gbq_without_dialect_warns_future_change
====== 2 passed in 15.00 seconds ======
```

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Hello @tswast! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/test_gbq.py`](https://github.com/tswast/pandas/blob/4115ef2c7f3ef41e45e80aebc0210cf1dafc8472/pandas/tests/io/test_gbq.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23303?src=pr&el=h1) Report
> Merging [#23303](https://codecov.io/gh/pandas-dev/pandas/pull/23303?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5a0c91e5a5bc61c71da08d6968d973255829678d?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23303/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23303?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23303   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         169      169           
  Lines       50897    50897           
=======================================
  Hits        46940    46940           
  Misses       3957     3957
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.65% <ø> (ø)` | :arrow_up: |
| #single | `42.29% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23303?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23303?src=pr&el=footer). Last update [5a0c91e...4115ef2](https://codecov.io/gh/pandas-dev/pandas/pull/23303?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 is the min version for pandas-gbq good in the install? I think the min version should be 0.6.0. Should I set that in the importorskip? I don't see versions for pandas-gbq anywhere when I search for it.

The last time we changed the arguments to read_gbq was https://github.com/pydata/pandas-gbq/pull/196, which shipped in 0.6.0 / 2018-08-15 and I updated pandas in https://github.com/pandas-dev/pandas/pull/22557 for that change. this is fine, we are only testing on a single build I think. We should move this to the 3.6 build. If you want to update the min version its ok by me (or not is fine too), I think mostly people just install the latest. thanks @tswast 