add clear both css after last column

git-svn-id: https://plugins.svn.wordpress.org/wens-responsive-column-layout-shortcodes/trunk@941074 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added clear div after the last coloumn

git-svn-id: https://plugins.svn.wordpress.org/wens-responsive-column-layout-shortcodes/trunk@941078 b8457f37-d9ea-0310-8a92-e5e31aec5664