avoid using PG_VERSION in a test-case (since global const was moved)

Author: kares

test/db/postgresql/simple_test.rb

@@ -59,7 +59,7 @@ def test_find_by_sql_WITH_statement
   end
 
   def test_create_xml_column
-    return unless PG_VERSION >= 80300
+    return if connection.postgresql_version < 80300
     super
   end if ar_version('3.1')
   def xml_sql_type; 'xml'; end