Log Message:
Update dovecot2-pigeonhole to 0.4.15.

v0.4.15 07-07-2016 Stephan Bosch <stephan@rename-it.nl>

* vacation extension: The sieve_user_email setting is now used in the check
  for implicit delivery.
- imapsieve plugin: For any mail transaction, the mailbox was opened a second
  time, even if no mailbox rule matched. This was unintentional, useless and
  caused problems when the imapsieve plugin was used with other plugins like
  acl.
- extprograms plugin: Significantly improved error handling. No stream errors
  were logged.
- extprograms plugin: Fixed bug in handling of result code from remote program
  (script service).
- extprograms plugin: Connection to remote program service was not retried.
- Several small fixes based on static analysis.
- Fixed handling of quoted string localparts in email addresses.