ElasticContainer: Use Elasticsearch 6.4.2 for V6_4 tests

Version 6.4.2 is the latest release on the 6.4.x series.

Change-Id: I5ac5d65dfa296d9735edf195239ce195cb514aea
diff --git a/gerrit-elasticsearch/src/test/java/com/google/gerrit/elasticsearch/ElasticContainer.java b/gerrit-elasticsearch/src/test/java/com/google/gerrit/elasticsearch/ElasticContainer.java
index 56c6e6b..bfeddd3 100644
--- a/gerrit-elasticsearch/src/test/java/com/google/gerrit/elasticsearch/ElasticContainer.java
+++ b/gerrit-elasticsearch/src/test/java/com/google/gerrit/elasticsearch/ElasticContainer.java
@@ -51,7 +51,7 @@
       case V6_3:
         return "docker.elastic.co/elasticsearch/elasticsearch-oss:6.3.2";
       case V6_4:
-        return "docker.elastic.co/elasticsearch/elasticsearch-oss:6.4.1";
+        return "docker.elastic.co/elasticsearch/elasticsearch-oss:6.4.2";
     }
     throw new IllegalStateException("No tests for version: " + version.name());
   }