prevent directory permission failure during the installation procedure

prefixing directory paths on the InstallRequirements->check() loop $this->requireWriteable(CIVICRM_DIRECTORY_SEPARATOR .$dir,...) is produce errors from failing the is_writable($dir) on WIN32 machines where the path is absolute and the prefix is dis-formed by the prefixing the path.
remove it from the parameter seams to clears the problem on my XP installation.