Fix a regression in PostgreSQL cookbook

Creation of 'travis' and 'rails' superusers was accidentally removed by
e4a893cbfc02f149476b21016ae3a785d660dca9 (see #221)

The new implementation relies on an SQL script, allowing Chef to get
notified if anything goes wrong on script execution.

Note that this script is designed to be executed only once
(during the first Chef provisioning run). It means that modifying
node.postgresql.superusers attribute won't be reflected in further runs.
[minor] Remove tab characters and fix indentation
PostgreSQL: improve System V service script

Return an error if requested PostgreSQL version doesn't exist.