update to Jetpack 2.9.3

git-svn-id: https://plugins.svn.wordpress.org/jetpack-sharing/trunk@892201 b8457f37-d9ea-0310-8a92-e5e31aec5664
add changelog line to readme

git-svn-id: https://plugins.svn.wordpress.org/jetpack-sharing/trunk@892202 b8457f37-d9ea-0310-8a92-e5e31aec5664