2.5.0 - Update release date in CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.5.0@1616280 13f79535-47bb-0310-9956-ffa450edef68
Reverting r1614132 (HDFS-6717) from branch-2.5.0 as this is only partial

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.5.0@1616282 13f79535-47bb-0310-9956-ffa450edef68