Updated CHANGES.txt to reflect hadoop-2.6.0 release dates.

(cherry picked from commit 770cc144425b6188d9b93178c77a35cd830a60df)
YARN-2834. Fixed ResourceManager to ignore token-renewal failures on recovery consistent with the (somewhat incorrect) behaviour in the non-recovery case. Contributed by Jian He.

(cherry picked from commit e76faebc9589654e83c8244ef9aff88391e56b80)
Fix broken download of directories

(cherry picked from commit 745c9a01243b8eefc72d89d1164d7d010b80983b)
Release Notes for hadoop-2.6.0.
Preparing to release hadoop-2.6.0: Set version in branch-2 to 2.7.0-SNAPSHOT.