fix(deps): update dependency nodemailer to v6

fix(deps): update dependency nodemailer to v6

This PR contains the following updates:

| Package | Type | Update | Change | References |
|---|---|---|---|---|
| nodemailer | dependencies | major | [`^5.0.0` -> `^6.0.0`](https://diff.intrinsic.com/nodemailer/5.1.1/6.0.0) | [homepage](https://nodemailer.com/), [source](https://togithub.com/nodemailer/nodemailer) |

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### Release Notes

<details>
<summary>nodemailer/nodemailer</summary>

### [`v6.0.0`](https://togithub.com/nodemailer/nodemailer/blob/master/CHANGELOG.md#&#8203;600-2019-03-25)

[Compare Source](https://togithub.com/nodemailer/nodemailer/compare/v5.1.1...v6.0.0)

-   SMTPConnection: use removeListener instead of removeAllListeners (xr0master) [ddc4af15]
    Using removeListener should fix memory leak with Node.js streams

</details>

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#283 automerged by dpebot

samples/package.json

@@ -17,7 +17,7 @@
     "@google-cloud/compute": "^0.12.0",
     "@sendgrid/mail": "^6.3.1",
     "node-fetch": "^2.3.0",
-    "nodemailer": "^5.0.0",
+    "nodemailer": "^6.0.0",
     "nodemailer-smtp-transport": "^2.7.4"
   },
   "devDependencies": {