Fixed CHANGES.txt to reflect that new content in branch-2 goes to hadoop-2.3.0.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1504743 13f79535-47bb-0310-9956-ffa450edef68
Set version in branch-2 to 2.3.0-SNAPSHOT to reflect that we will only be adding new content to hadoop-2.3.0 and not hadoop-2.2.0.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1504744 13f79535-47bb-0310-9956-ffa450edef68