OAK-1822: NodeDocument _modified may go back in time

Use MongoDB 2.6.1 on travis and re-apply fix

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1601838 13f79535-47bb-0310-9956-ffa450edef68
OAK-301 : clarification about the property index, index inspection options

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1601843 13f79535-47bb-0310-9956-ffa450edef68