Preparing to release hadoop-2.4.1.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1602888 13f79535-47bb-0310-9956-ffa450edef68
Set version in branch-2.4 to 2.4.2-SNAPSHOT.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1602895 13f79535-47bb-0310-9956-ffa450edef68