Set the release date for 2.7.4

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -1,6 +1,6 @@
 Hadoop Change Log
 
-Release 2.7.4 - UNRELEASED
+Release 2.7.4 - 2017-08-04
 
   INCOMPATIBLE CHANGES
 

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -1,6 +1,6 @@
 Hadoop HDFS Change Log
 
-Release 2.7.4 - UNRELEASED
+Release 2.7.4 - 2017-08-04
 
   INCOMPATIBLE CHANGES
 

hadoop-mapreduce-project/CHANGES.txt

@@ -1,6 +1,6 @@
 Hadoop MapReduce Change Log
 
-Release 2.7.4 - UNRELEASED
+Release 2.7.4 - 2017-08-04
 
   INCOMPATIBLE CHANGES
 

hadoop-yarn-project/CHANGES.txt

@@ -1,6 +1,6 @@
 Hadoop YARN Change Log
 
-Release 2.7.4 - UNRELEASED
+Release 2.7.4 - 2017-08-04
 
   INCOMPATIBLE CHANGES