imapfilter: use TLS1.2 with fastmail

Works with the latest version of imapfilter.

Note that this new version finally supports certificate
validation via the standard paths -- no more manual
deletion of ~/.imapfilter/certificates!

Signed-off-by: Emilio G. Cota <cota@braap.org>
Merge branch 'master' into flamenco

Signed-off-by: Emilio G. Cota <cota@braap.org>