Upgrade to AsciidoctorJ 2.5.4

Closes gh-835

Author: wilkinsona

spring-restdocs-platform/build.gradle

@@ -14,7 +14,7 @@ dependencies {
 		api("junit:junit:4.12")
 		api("io.rest-assured:rest-assured:5.0.1")
 		api("org.apache.pdfbox:pdfbox:2.0.7")
-		api("org.asciidoctor:asciidoctorj:2.5.2")
+		api("org.asciidoctor:asciidoctorj:2.5.4")
 		api("org.asciidoctor:asciidoctorj-pdf:1.6.0")
 		api("org.assertj:assertj-core:3.11.1")
 		api("org.hamcrest:hamcrest-core:1.3")