chore(deps): update dependency google-cloud-dataproc to v0.8.1 (#4015)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-dataproc](https://togithub.com/googleapis/python-dataproc) | patch | `==0.8.0` -> `==0.8.1` |

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### Release Notes

<details>
<summary>googleapis/python-dataproc</summary>

### [`v0.8.1`](https://togithub.com/googleapis/python-dataproc/blob/master/CHANGELOG.md#&#8203;081-httpswwwgithubcomgoogleapispython-dataproccomparev080v081-2020-06-05)

[Compare Source](https://togithub.com/googleapis/python-dataproc/compare/v0.8.0...v0.8.1)

</details>

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dataproc/requirements.txt

@@ -3,4 +3,4 @@ google-auth==1.16.1
 google-auth-httplib2==0.0.3
 google-cloud==0.34.0
 google-cloud-storage==1.28.1
-google-cloud-dataproc==0.8.0
+google-cloud-dataproc==0.8.1