Downgrade to spring-boot-gradle-plugin:1.5.2

Fixes gh-5829

build.gradle

@@ -11,7 +11,7 @@ buildscript {
 		classpath('me.champeau.gradle:gradle-javadoc-hotfix-plugin:0.1')
 		classpath('org.asciidoctor:asciidoctor-gradle-plugin:1.5.1')
 		classpath("io.spring.gradle:docbook-reference-plugin:0.3.1")
-		classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.15.RELEASE")
+		classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.2.RELEASE")
 	}
 }