puppet: Run thumbor by default on voyager systems.

With this change, all that one needs to do to start using thumbor in
production is to set the `THUMBOR_URL` setting.

Since without THUMBOR_URL enabled, the thumbor service doesn't
actually do anything, this is pretty safe.

puppet/zulip/manifests/dockervoyager.pp

@@ -9,6 +9,7 @@ class zulip::dockervoyager {
   include zulip::app_frontend
   include zulip::supervisor
   include zulip::process_fts_updates
+  include zulip::thumbor
 
   file { '/etc/supervisor/conf.d/cron.conf':
     ensure  => file,

puppet/zulip/manifests/voyager.pp

@@ -19,4 +19,5 @@ class zulip::voyager {
   include zulip::redis
   include zulip::localhost_camo
   include zulip::static_asset_compiler
+  include zulip::thumbor
 }