Remove version 2.4 support. (apache#180)

Author: triclambert

docs/version-policy.md

@@ -5,4 +5,4 @@ feature_maturity: stable
 enterprise: 'no'
 ---
 
-We have selected the latest version of the [Apache Spark](http://spark.apache.org) stable release train for new releases. We support HDFS version 2.6 by default and versions 2.4 and 2.7 with customization.
+We have selected the latest version of the [Apache Spark](http://spark.apache.org) stable release train for new releases. We support HDFS version 2.6 by default and version 2.7.