Use /usr/bin/env python for process_fts_updates.

`/usr/bin/env python` is almost always preferred over specifying the
specific python to run (and this script doesn't work for me on OSX
with /usr/bin/python specified).

(imported from commit 531e6062ba0ac1f25e3c681bb5cf83a918d0e3e7)

puppet/zulip/files/postgresql/process_fts_updates

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 import psycopg2
 import psycopg2.extensions
 import select