Deprecate Postgresql versions older then 8.4

This patch adds a deprecation warning at PuppetDB startup if the user is using
a version of PostgreSQL older than 8.4.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #390 from kbarber/ticket/master/postgresql-8x-deprecation

DONT MERGE YET: Deprecate Postgresql versions older then 8.4