[Dovecot] v2.1.15 released

[Timo Sirainen]

http://dovecot.org/releases/2.1/dovecot-2.1.15.tar.gz
http://dovecot.org/releases/2.1/dovecot-2.1.15.tar.gz.sig

v2.1.14's dovecot.index.cache fixes caused Dovecot to use more disk I/O
and memory than was necessary.