package/{dovecot, dovecot-pigeonhole}: bump version to 2.3.4, 0.5.4

package/{dovecot, dovecot-pigeonhole}: bump version to 2.3.4, 0.5.4

We need to bump both packages in one commit:

https://dovecot.org/pipermail/dovecot-news/2018-November/000392.html

 Adjustments to several changes in Dovecot v2.3.4 make this Pigeonhole
 release dependent on that Dovecot release; it will not compile against
 older Dovecot versions. And, conversely, you need to upgrade
 Pigeonhole when upgrading Dovecot to v2.3.4.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>