Added Support for Ubuntu 15.04 in OS detect code.

Till Brehm

2015-05-06 ed46b479b558fd5dba53f4bf79caf360130ba4ce

 server/plugins-available/mail_plugin_dkim.inc.php

@@ -75,9 +75,9 @@
    */
   function get_amavis_config() {
      $pos_config=array(
         '/etc/amavisd.conf',
         '/etc/amavisd.conf/50-user',
         '/etc/amavis/conf.d/50-user',
         '/etc/amavisd.conf',
         '/etc/amavisd/amavisd.conf'
      );
      $amavis_configfile='';