Upgrade to AsciidoctorJ PDF 1.5.0-alpha.18

Closes gh-622

Author: wilkinsona

build.gradle

@@ -83,7 +83,7 @@ subprojects {
 			dependency 'org.apache.pdfbox:pdfbox:2.0.7'
 			dependency 'org.assertj:assertj-core:2.9.1'
 			dependency 'org.asciidoctor:asciidoctorj:1.5.8.1'
-			dependency 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16'
+			dependency 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.18'
 			dependency 'org.hamcrest:hamcrest-core:1.3'
 			dependency 'org.hamcrest:hamcrest-library:1.3'
 			dependency 'org.hibernate:hibernate-validator:5.2.5.Final'