pgsql: IF NOT EXISTS is only supported since PostgreSQL 9.1

Clicking on "Install SQL table" a second time will error
without damages

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Merge pull request #369 from echampet/pgsql-create

pgsql: IF NOT EXISTS is only supported since PostgreSQL 9.1