(PDB-79) Drop support for Postgres < 8.4

PuppetDB will now log an error and exit if it connects to an instance of Postgres
older than 8.4. Users of older versions will need to upgrade (especially EL 5
users as it defaults to 8.1).  The acceptance tests for EL 5 have been updated
to be explicit about using Postgres 8.4 packages instead.