Bump nodemailer from 4.0.1 to 6.4.16

Bumps [nodemailer](https://github.com/nodemailer/nodemailer) from 4.0.1 to 6.4.16.
- [Release notes](https://github.com/nodemailer/nodemailer/releases)
- [Changelog](https://github.com/nodemailer/nodemailer/blob/master/CHANGELOG.md)
- [Commits](nodemailer/nodemailer@v4.0.1...v6.4.16)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -28,7 +28,7 @@
     "moment": "^2.18.0",
     "mongoose": "^4.9.2",
     "morgan": "~1.8.1",
-    "nodemailer": "^4.0.1",
+    "nodemailer": "^6.4.16",
     "qn": "^0.2.2",
     "qs": "^2.3.3",
     "sendmail": "^1.1.1",