update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/jetpack-widget-visibility/trunk@823022 b8457f37-d9ea-0310-8a92-e5e31aec5664
update to 2.7

git-svn-id: https://plugins.svn.wordpress.org/jetpack-widget-visibility/trunk@823993 b8457f37-d9ea-0310-8a92-e5e31aec5664