ElasticContainer: Update the 5.6 version to latest

There were no breaking changes since the previously used version,
according to the release notes [1] and [2]. Considering the latter as
well since the former was just an overall Elastic stack version update
for Elasticsearch.

[1] https://www.elastic.co/guide/en/elasticsearch/reference/5.6/release-notes-5.6.15.html
[2] https://www.elastic.co/guide/en/elasticsearch/reference/5.6/release-notes-5.6.14.html

Change-Id: I7fe9f99265958f8d600c982e131134c3b5a484db
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 0b6e3ee..9bdf4eb 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
@@ -46,7 +46,7 @@
       case V2_4:
         return "elasticsearch:2.4.6-alpine";
       case V5_6:
-        return "docker.elastic.co/elasticsearch/elasticsearch:5.6.13";
+        return "docker.elastic.co/elasticsearch/elasticsearch:5.6.15";
       case V6_2:
         return "docker.elastic.co/elasticsearch/elasticsearch-oss:6.2.4";
       case V6_3: