Avoid breaking Stoq using PostgreSQL <= 8.4

PostgreSQL <= 8.4 doesn't have a pg_available_extensions table. On that
situation, a ProgrammingError is raised. To avoid breaking a lot of
installations without a good reason, except that error and warn the user that
he should migrate it's database.

Change-Id: I49e47f82cf98f83970aa4247257ee5b8515d3bdc