Pump asciidoctor version

- New versions needed for jdk11
- Relates to #863

build.gradle

@@ -23,8 +23,8 @@ buildscript {
 	}
 	dependencies {
 		classpath("io.spring.gradle:propdeps-plugin:0.0.8")
-		classpath('org.asciidoctor:asciidoctor-gradle-plugin:1.5.6')
-		classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16'
+		classpath('org.asciidoctor:asciidoctor-gradle-plugin:1.6.1')
+		classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.3'
 		classpath("org.springframework.boot:spring-boot-gradle-plugin:$springBootVersion")
 	}
 }