How to change pg_trigger query so that it works in 9.0 without pg_trigger.tgisconstraint does not exist error

In 9.0 query below returns error

7/42703:ERROR: column pg_trigger.tgisconstraint does not exist

How to change it so that it works in all servers starting at 8.0 (or at
least
from 8.1) to 9.0 ?
Or if this is not possible how to fix it so that it works in 9 ?

Andrus

SELECT
  pg_catalog.pg_get_triggerdef(pg_trigger.oid) as trdef
FROM pg_catalog.pg_trigger
join pg_catalog.pg_class on pg_trigger.tgrelid = pg_class.oid
JOIN pg_catalog.pg_namespace ON pg_namespace.oid=pg_class.relnamespace
WHERE not pg_trigger.tgisconstraint
and pg_namespace.nspname='firma1'