ElasticContainer: Use Elasticsearch 6.4.2 for V6_4 tests

This is a cherry-pick of stable-2.15 recently merged commit fc31842.
This pick assumes that Elasticsearch 6.x.y is still also maintained for
the stable-2.14 branch.

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

Change-Id: I615aa08b1bf5d924022ef421cb9ed35e7154068c
diff --git a/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java b/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java
index 4e95674..e347f75 100644
--- a/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java
+++ b/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java
@@ -52,7 +52,7 @@
       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());
   }