package/simple-mail: version bump 3.1.0
For change log since 3.0.0, see:
https://github.com/cutelyst/simple-mail/compare/v3.0.0...v3.1.0
Signed-off-by: Zoltan Gyarmati <zgyarmati@zgyarmati.de>
[Julien: add link to change log]
Signed-off-by: Julien Olivain <ju.o@free.fr>