Preparing for release 2.6.0

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1607539 13f79535-47bb-0310-9956-ffa450edef68
Preparing for 2.6.0 development. Updated versions.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1607541 13f79535-47bb-0310-9956-ffa450edef68