Deprecated dependencies

`nodemailer` should be updated:

```
warning bucker > nodemailer@0.5.15: All versions below 4.0.1 of Nodemailer are deprecated. See https://nodemailer.com/status/
warning bucker > nodemailer > mailcomposer > mimelib@0.2.19: This module is deprecated
warning bucker > nodemailer > mailcomposer > dkim-signer > mimelib@0.2.19: This module is deprecated
```