Update dovecot version from wheezy backports

For correct implementation of the fix for logjam attack (https://github.com/sovereign/sovereign/pull/372), state=latest is needed to grab sufficient version of Dovecot. If not then 37aa7e2cb5 doesn't work.

roles/mailserver/tasks/dovecot.yml

@@ -3,7 +3,7 @@
   when: ansible_distribution_release == 'wheezy'
 
 - name: Install Dovecot and related packages on Debian 7
-  apt: pkg={{ item }} update_cache=yes state=installed default_release=wheezy-backports
+  apt: pkg={{ item }} update_cache=yes state=latest default_release=wheezy-backports
   with_items:
     - dovecot-core
     - dovecot-imapd