[fix #5173] JRouterInstallation::build() strict standards

https://github.com/joomla/joomla-cms/pull/5173 corrected parse() and broke build()

To test, install a new staging and make sure you do not get the error:
[25-Nov-2014 09:47:40 UTC] PHP Strict standards: Declaration of JRouterInstallation::build() should be compatible with JRouter::build($url) in ROOT/installation/application/router.php on line 47