Update dependency google-cloud-kms to v1.4.0 (#3410)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-kms](https://togithub.com/googleapis/python-kms) | minor | `==1.3.0` -> `==1.4.0` |

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### Release Notes

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

### [`v1.4.0`](https://togithub.com/googleapis/python-kms/blob/master/CHANGELOG.md#&#8203;140-httpswwwgithubcomgoogleapispython-kmscomparev130v140-2020-04-14)

[Compare Source](https://togithub.com/googleapis/python-kms/compare/v1.3.0...v1.4.0)

##### Features

-   add support for external key manager (via synth) ([#&#8203;8](https://www.github.com/googleapis/python-kms/issues/8)) ([4077fc8](https://www.github.com/googleapis/python-kms/commit/4077fc89943cc09d489d44c05efcf9cab61cdbaf))

</details>

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kms/api-client/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-kms==1.3.0
+google-cloud-kms==1.4.0
 cryptography==2.8