Remove references to Debian 7

CONTRIBUTING.md

 Make sure you agree with the license (GPLv3). See [LICENSE.md](./LICENSE.md) for details.
 
 If you issue a pull request, please specify what distribution you used for testing (if any).
-Code that is committed to the master branch should work with both Debian 7 and Ubuntu 14.04 LTS (Debian 8 support is coming up).
+Code that is committed to the master branch should work with both Debian 8 "Jessie" and Ubuntu 14.04 LTS "Trusty".

roles/mailserver/tasks/dovecot.yml

-- name: Install Dovecot and related packages on distributions other than Debian 7
+- name: Install Dovecot and related packages
   apt: pkg={{ item }} update_cache=yes state=installed
   with_items:
     - dovecot-core