Upgrade to Elasticsearch 5.6.15

Closes gh-16123

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -60,7 +60,7 @@
 		<embedded-mongo.version>2.0.3</embedded-mongo.version>
 		<flyway.version>5.0.7</flyway.version>
 		<freemarker.version>2.3.28</freemarker.version>
-		<elasticsearch.version>5.6.14</elasticsearch.version>
+		<elasticsearch.version>5.6.15</elasticsearch.version>
 		<glassfish-el.version>3.0.0</glassfish-el.version>
 		<groovy.version>2.4.16</groovy.version>
 		<gson.version>2.8.5</gson.version>