Bump nodemailer from 4.7.0 to 6.4.16

Bumps [nodemailer](https://github.com/nodemailer/nodemailer) from 4.7.0 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@v4.7.0...v6.4.16)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -19,7 +19,7 @@
     "moment": "^2.18.1",
     "mongoose": "^5.7.7",
     "morgan": "~1.9.1",
-    "nodemailer": "^4.0.1",
+    "nodemailer": "^6.4.16",
     "passport": "^0.3.2",
     "passport-github": "^1.1.0",
     "passport-heroku": "^1.0.0",