reenable hack to use CURRENT_TIMESTAMP for postgresql when creating the database, fixes oc-1852

TODO for OC5: use CURRENT_TIMESTAMP for all databases. mysql supports it as a default for DATETIME since 5.6.5 [1]
as a fallback we could use <default>0000-01-01 00:00:00</default> everywhere
[1] http://bugs.mysql.com/bug.php?id=27645
http://dev.mysql.com/doc/refman/5.0/en/timestamp-initialization.html
http://www.postgresql.org/docs/8.1/static/functions-datetime.html
http://www.sqlite.org/lang_createtable.html
http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions037.htm