fix(deps): update dependency @google-cloud/promisify to v2 (#928)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [@google-cloud/promisify](https://github.com/googleapis/nodejs-promisify) | dependencies | major | [`^1.0.0` -> `^2.0.0`](https://renovatebot.com/diffs/npm/@google-cloud%2fpromisify/1.0.4/2.0.0) |

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### Release Notes

<details>
<summary>googleapis/nodejs-promisify</summary>

### [`v2.0.0`](https://github.com/googleapis/nodejs-promisify/blob/master/CHANGELOG.md#&#8203;200-httpswwwgithubcomgoogleapisnodejs-promisifycomparev104v200-2020-03-23)

[Compare Source](https://github.com/googleapis/nodejs-promisify/compare/v1.0.4...v2.0.0)

##### ⚠ BREAKING CHANGES

-   update to latest version of gts/typescript ([#&#8203;183](https://github.com/googleapis/nodejs-promisify/issues/183))
-   drop Node 8 from engines field ([#&#8203;184](https://github.com/googleapis/nodejs-promisify/issues/184))

##### Features

-   drop Node 8 from engines field ([#&#8203;184](https://www.github.com/googleapis/nodejs-promisify/issues/184)) ([7e6d3c5](https://www.github.com/googleapis/nodejs-promisify/commit/7e6d3c54066d89530ed25c7f9722efd252f43fb8))

##### Build System

-   update to latest version of gts/typescript ([#&#8203;183](https://www.github.com/googleapis/nodejs-promisify/issues/183)) ([9c3ed12](https://www.github.com/googleapis/nodejs-promisify/commit/9c3ed12c12f4bb1e17af7440c6371c4cefddcd59))

##### [1.0.4](https://www.github.com/googleapis/nodejs-promisify/compare/v1.0.3...v1.0.4) (2019-12-05)

##### Bug Fixes

-   **deps:** pin TypeScript below 3.7.0 ([e48750e](https://www.github.com/googleapis/nodejs-promisify/commit/e48750ef96aa20eb3a2b73fe2f062d04430468a7))

##### [1.0.3](https://www.github.com/googleapis/nodejs-promisify/compare/v1.0.2...v1.0.3) (2019-11-13)

##### Bug Fixes

-   **docs:** add jsdoc-region-tag plugin ([#&#8203;146](https://www.github.com/googleapis/nodejs-promisify/issues/146)) ([ff0ee74](https://www.github.com/googleapis/nodejs-promisify/commit/ff0ee7408f50e8f7147b8ccf7e10337aa5920076))

##### [1.0.2](https://www.github.com/googleapis/nodejs-promisify/compare/v1.0.1...v1.0.2) (2019-06-26)

##### Bug Fixes

-   **docs:** link to reference docs section on googleapis.dev ([#&#8203;128](https://www.github.com/googleapis/nodejs-promisify/issues/128)) ([5a8bd90](https://www.github.com/googleapis/nodejs-promisify/commit/5a8bd90))

##### [1.0.1](https://www.github.com/googleapis/nodejs-promisify/compare/v1.0.0...v1.0.1) (2019-06-14)

##### Bug Fixes

-   **docs:** move to new client docs URL ([#&#8203;124](https://www.github.com/googleapis/nodejs-promisify/issues/124)) ([34d18cd](https://www.github.com/googleapis/nodejs-promisify/commit/34d18cd))

</details>

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package.json

@@ -51,7 +51,7 @@
     "@google-cloud/paginator": "^2.0.0",
     "@google-cloud/precise-date": "^2.0.0",
     "@google-cloud/projectify": "^1.0.0",
-    "@google-cloud/promisify": "^1.0.0",
+    "@google-cloud/promisify": "^2.0.0",
     "@types/duplexify": "^3.6.0",
     "@types/long": "^4.0.0",
     "arrify": "^2.0.0",