Set the release date for 2.5.0 in CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1619237 13f79535-47bb-0310-9956-ffa450edef68
YARN-1919. Potential NPE in EmbeddedElectorService#stop. (Tsuyoshi Ozawa via kasha)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1619251 13f79535-47bb-0310-9956-ffa450edef68