ElasticContainer: Bump the test server version to 5.6.15

According to the release notes [1], there are no changes in this new
version. It is likely just to align with other Elastic stack update(s).

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

Change-Id: I345e9121a45a6c478a3b3707409924ad232d0239
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 454bffd..b4ca6d8 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
@@ -37,7 +37,7 @@
   private static String getImageName(ElasticVersion version) {
     switch (version) {
       case V5_6:
-        return "docker.elastic.co/elasticsearch/elasticsearch:5.6.14";
+        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: