Expect two node tags only in >= 9.4
Don't compile the guts of the ddl_command_end trigger for < 9.5

Both pg_event_trigger_get_creation_commands() and
pg_event_trigger_expand_command() are in the latest deparse branch.

We create the trigger for older versions anyway, so that we don't need
to try to make the "CREATE EXTENSION pgaudit" SQL do something different
based on the version.