fix(update-ngxblocker): use correct unix mail return address flag

[bfahrenfort]

update-ngxblocker uses -f to specify the return address. GNU mail.mailutils uses -r to specify the return address.

previously, update-ngxblocker would report:

Emailing report to: <snip>

mail: conflicting options

The second line is gone with this change, and /var/log/mail.log reports (250 Ok).

You still specify the right return address with the same command, update-ngxblocker -e <notification email address> -f <send address> [other mailgun args, if using mailgun].

Fixes #517.

[marbens-arch]

Maybe you should edit this PR to add "Fixes #517." to the end of the description, because it does.