Deprecate PG 9.1 instead, and remove nagging about 9.3 for PE users

This changes the deprecation to 9.1 and below. It also removes the nagging about
upgrading for PE users, who will still be on 9.2 it looks like.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #1064 from kbarber/maint/master/change-postgresql-deprecation-version

Deprecate PG 9.1 instead, and remove nagging about 9.3 for PE users