libmail-dkim-perl added to vst-install-debian.sh

According to issue https://github.com/serghey-rodin/vesta/issues/870

install/vst-install-debian.sh

@@ -29,7 +29,7 @@ if [ "$release" -eq 8 ]; then
         mysql-client postgresql postgresql-contrib phppgadmin phpMyAdmin mc
         flex whois rssh git idn zip sudo bc ftp lsof ntpdate rrdtool quota
         e2fslibs bsdutils e2fsprogs curl imagemagick fail2ban dnsutils
-        bsdmainutils cron vesta vesta-nginx vesta-php expect"
+        bsdmainutils cron vesta vesta-nginx vesta-php expect libmail-dkim-perl"
 else
     software="nginx apache2 apache2-utils apache2.2-common
         apache2-suexec-custom libapache2-mod-ruid2