Make python-postmonkey not a required package.

This is in some ways a regression, but because we don't have
python-postmonkey packaged right now, this is required to make the
Zulip production installation process work on Trusty.

(imported from commit 539d253eb7fedc20bf02cc1f0674e9345beebf48)

puppet/zulip/manifests/app_frontend.pp

@@ -48,7 +48,6 @@ class zulip::app_frontend {
                     # Needed for S3 file uploads
                     "python-boto",
                     # Needed to send email
-                    "python-postmonkey",
                     "python-mandrill",
                     # Needed to generate diffs for edits
                     "python-diff-match-patch",