Remove extra br tag for IE9/10

http://www.tinymce.com/develop/bugtracker_view.php?id=6610
Only add a bogus br tag on empty editors for IE < 9 instead of IE < 11,
as IE9 and IE10 render the elements properly and don't need the extra
tag.
Merge branch 'fixIEDoubleLine' of https://github.com/monkeydiane/tinymce into fixIEDoubleLine