[#33688] JApplicationCms::getRouter is a static method

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33688&start=0

We need to grab the JRouterSite/JRouterAdministrator object in various places in Joomla. And currently the function is a static function. But we're not calling it statically - and with the PHPUnit upgrade travis has just done that's suddenly become important as we can't mock the static methods. So we now *must* call this correctly for the unit tests to function (see https://github.com/joomla/joomla-cms/pull/3550)

## Testing
- Check that routing in frontend & backend still work (SEF URLs on & off)
- Check the com_finder frontend search view still works and generates the correct URLS
- Check internal links (that would use JRoute) work as expected. i.e. going from category view to single article view.
- Check login module redirection url works
- Check language filter plugin still works