Mark as 2.6.8

git-svn-id: https://plugins.svn.wordpress.org/query-monitor/trunk@942858 b8457f37-d9ea-0310-8a92-e5e31aec5664
Readme update

git-svn-id: https://plugins.svn.wordpress.org/query-monitor/trunk@942859 b8457f37-d9ea-0310-8a92-e5e31aec5664