Bump nodemailer from 2.7.2 to 6.4.16

Bumps [nodemailer](https://github.com/nodemailer/nodemailer) from 2.7.2 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@v2.7.2...v6.4.16)

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

Author: dependabot[bot]

package-lock.json

@@ -188,7 +188,7 @@
     "mockingoose": "^2.13.2",
     "mongoose": "^5.12.3",
     "node-uuid": "^1.4.7",
-    "nodemailer": "^2.6.4",
+    "nodemailer": "^6.4.16",
     "nodemailer-mailgun-transport": "^1.4.0",
     "passport": "^0.3.2",
     "passport-github": "^1.1.0",