Update Asciidoctor, plugin, and dependencies to latest versions

The plugin and Asciidoctor need to be updated at the same time, because
plugin versions 3.x only support Asciidoctor 2.x and older plugin
versions only supports Asciidoctor 1.x.

build.gradle

@@ -14,7 +14,7 @@ buildscript {
 }
 
 plugins {
-    id "org.asciidoctor.jvm.convert" version "2.4.0"
+    id "org.asciidoctor.jvm.convert" version "3.1.0"
     id "com.github.jruby-gradle.base" version "1.7.0"
 }
 
@@ -34,15 +34,15 @@ configurations {
 }
 
 dependencies {
-    asciidoctorDeps 'org.asciidoctor:asciidoctorj-diagram:1.5.18'
-    asciidoctorDeps 'org.asciidoctor:asciidoctorj-epub3:1.5.0-alpha.9'
-    asciidoctorDeps 'org.asciidoctor:asciidoctorj-pdf:1.5.0-beta.8'
+    asciidoctorDeps 'org.asciidoctor:asciidoctorj-diagram:2.1.0'
+    asciidoctorDeps 'org.asciidoctor:asciidoctorj-epub3:1.5.0-alpha.19'
+    asciidoctorDeps 'org.asciidoctor:asciidoctorj-pdf:1.5.4'
     
     gems 'rubygems:asciidoctor-bibtex:0.8.0'
 }
 
 asciidoctorj {
-    version = '1.6.2'
+    version = '2.4.2'
 }
 
 task bibtex() {