Set the release date for 2.5.0 in CHANGES.txt

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YARN-1919. Potential NPE in EmbeddedElectorService#stop. (Tsuyoshi Ozawa via kasha)

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HDFS-6858. Merging change r1619256 from trunk to branch-2.

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HDFS-6878: Merging r1619271 from trunk to branch-2.

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HDFS-6758: Merging r1619275 from trunk to branch-2.

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