puppet: Add gettext to dependencies for app instances.

This is already installed on a lot of systems, and is used indirectly
when upgrading Zulip from Git.

We previously removed this in
263212decf, I believe due to an
incorrect understanding of only makemessages needing it.

puppet/zulip/manifests/app_frontend_base.pp

@@ -16,6 +16,8 @@ class zulip::app_frontend_base {
   }
   zulip::safepackage {
     [
+      # For `manage.py compilemessages` when upgrading from Git.
+      'gettext',
       # For Slack import.
       'unzip',
       # Ensures `/etc/ldap/ldap.conf` exists; the default