Fix pandas-gbq deps
Should close #25910, see https://github.com/pandas-dev/pandas/issues/25910#issuecomment-477611959. @jreback 
As a side note, it seem the gbq-tests are not actually run correctly in PRs, otherwise this should have been caught in #25554.  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25912?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@68dd979`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25912/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25912?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #25912   +/-   ##
=========================================
  Coverage          ?   91.55%           
=========================================
  Files             ?      175           
  Lines             ?    52775           
  Branches          ?        0           
=========================================
  Hits              ?    48320           
  Misses            ?     4455           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.12% <ø> (?)` | |
| #single | `41.82% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25912?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/25912?src=pr&el=footer). Last update [68dd979...417454f](https://codecov.io/gh/pandas-dev/pandas/pull/25912?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25912?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@68dd979`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25912/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25912?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #25912   +/-   ##
=========================================
  Coverage          ?   91.55%           
=========================================
  Files             ?      175           
  Lines             ?    52775           
  Branches          ?        0           
=========================================
  Hits              ?    48320           
  Misses            ?     4455           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.12% <ø> (?)` | |
| #single | `41.82% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25912?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/25912?src=pr&el=footer). Last update [68dd979...417454f](https://codecov.io/gh/pandas-dev/pandas/pull/25912?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @h-vetinari  > @WillAyd: Makes sense to me. I assume can remove once 0.9.1 comes out right?

It's actually been fixed by @tswast in 0.9.0 [already](https://github.com/pandas-dev/pandas/issues/25910#issuecomment-477680482), so, if I understand correctly, this PR could safely be reverted now.