allows 'none' as authentication mode

Fixes Packager wizard smtp addon to support 'none' as authentication mode.
What do you think about this shorter version? It might not be as explizit as yours but it is a little bit shorter.

    smtp_authentication = ENV.fetch('SMTP_AUTHENTICATION', none)
    smtp_authentication = smtp_authentication.to_sym
    smtp_authentication = nil if smtp_authentication == "none"