frontend url build in joomla

in joomla.php we need to modify the frontend url hack in v4.0
see this thread: http://forum.civicrm.org/index.php/topic,6454.0.html

J1.6 does not use an index2.php file, but civi frontend urls do need to be rewritten without the administrator/ path.
to avoid divergent versions we could just do a string replace on both administrator/index2.php and administrator/index.php