ENH: update pandas-gbq to 0.8.0, adds credentials arg
- N/A closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @tswast! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/tswast/pandas/blob/a3f97983757763815413d6cb9bb68add7c3fc680/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/io/gbq.py`](https://github.com/tswast/pandas/blob/a3f97983757763815413d6cb9bb68add7c3fc680/pandas/io/gbq.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/test_gbq.py`](https://github.com/tswast/pandas/blob/a3f97983757763815413d6cb9bb68add7c3fc680/pandas/tests/io/test_gbq.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23662?src=pr&el=h1) Report
> Merging [#23662](https://codecov.io/gh/pandas-dev/pandas/pull/23662?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4b945adad587851d570e950d05a1da75c5dc76c?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/23662/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23662?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23662   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51339    51339           
=======================================
  Hits        47360    47360           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23662?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/23662/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23662/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23662?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/23662?src=pr&el=footer). Last update [b4b945a...d633cdf](https://codecov.io/gh/pandas-dev/pandas/pull/23662?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. @WillAyd over to you. Thanks @tswast 