Upgrade to AsciidoctorJ PDF 2.1.4

Closes gh-837

Author: wilkinsona

spring-restdocs-platform/build.gradle

@@ -15,7 +15,7 @@ dependencies {
 		api("io.rest-assured:rest-assured:5.1.1")
 		api("org.apache.pdfbox:pdfbox:2.0.7")
 		api("org.asciidoctor:asciidoctorj:2.5.4")
-		api("org.asciidoctor:asciidoctorj-pdf:1.6.0")
+		api("org.asciidoctor:asciidoctorj-pdf:2.1.4")
 		api("org.assertj:assertj-core:3.11.1")
 		api("org.hamcrest:hamcrest-core:1.3")
 		api("org.hamcrest:hamcrest-library:1.3")