Jetpack: Handle post-merge 3.8-alpha stylings where MP6 is merged in and no longer provides the body class. In the admin, provide a 
.pre-mp6 body class if the user is on 3.7 or lower.


git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@807747 b8457f37-d9ea-0310-8a92-e5e31aec5664