The Mail gem changed format of the delivery method arguments for
sendmail from a string to an array of strings in this commit
https://github.com/mikel/mail/commit/7e1196bd29815a0901d7290c82a332c0959b163a

As the settings are now a sendmail delivery method produces the
following error:
```
.../mail-2.8.0/lib/mail/network/delivery_methods/sendmail.rb:53:in `initialize': :arguments expected to be an Array of individual string args (ArgumentError)
```

This also updates the mail dependency since the new default won't work
with the older versions.