psql: server version check for \dO

Hi all,

I think psql's \dO command is missing the server version check which
similar commands such as \dx use. Right now \dO errors out with:

test=# \dO
ERROR:  relation "pg_catalog.pg_collation" does not exist

when talking to servers older than 9.1, which don't have pg_collation.
Simple patch for listCollations() attached.

Josh