Update to spring-boot-gradle-plugin 1.5.18

Fixes gh-6408

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:1.5.17.RELEASE")
+		classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.18.RELEASE")
 	}
 }