Asciidoctor-ant and asciidoctor-bibtex

I get this exception when I use asciidoctor-bibtex from asciidoctor-ant:

>ant withextension

withextension:
[asciidoctor:convert] Ignoring nokogiri-1.7.1 because its extensions are not built.  Try: gem pristine nokogiri --version 1.7.1
[asciidoctor:convert] Ignoring unicode-0.4.4.4 because its extensions are not built.  Try: gem pristine unicode --version 0.4.4.4
[asciidoctor:convert] UTF-8
[asciidoctor:convert] Render bibtext.adoc from doc to build with backend=html
[asciidoctor:convert] Aug 03, 2017 10:49:32 PM org.asciidoctor.internal.JRubyAsciidoctor convertFile
[asciidoctor:convert] SEVERE: (NameError) asciidoctor: FAILED: /home/rocky/tabletest/doc/bibtext.adoc: Failed to load AsciiDoc document - TreeProcessor not found in packages org.asciidoctor.extension; last error: cannot load Java class org.asciidoctor.extension.TreeProcessor

BUILD FAILED
/home/rocky/tabletest/build.xml:33: org.asciidoctor.internal.AsciidoctorCoreException: org.jruby.exceptions.RaiseException: (NameError) asciidoctor: FAILED: /home/rocky/tabletest/doc/bibtext.adoc: Failed to load AsciiDoc document - TreeProcessor not found in packages org.asciidoctor.extension; last error: cannot load Java class org.asciidoctor.extension.TreeProcessor
        at org.asciidoctor.internal.JRubyAsciidoctor.convertFile(JRubyAsciidoctor.java:526)
        at org.asciidoctor.internal.JRubyAsciidoctor.convertFile(JRubyAsciidoctor.java:498)
        at org.asciidoctor.internal.JRubyAsciidoctor.renderFile(JRubyAsciidoctor.java:252)
        at org.asciidoctor.internal.JRubyAsciidoctor.renderFile(JRubyAsciidoctor.java:337)
        at org.asciidoctor.ant.AsciidoctorAntTask.execute(AsciidoctorAntTask.java:95)
        at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:293)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
        at org.apache.tools.ant.Task.perform(Task.java:348)
        at org.apache.tools.ant.Target.execute(Target.java:435)
        at org.apache.tools.ant.Target.performTasks(Target.java:456)
        at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1405)
        at org.apache.tools.ant.Project.executeTarget(Project.java:1376)
        at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
        at org.apache.tools.ant.Project.executeTargets(Project.java:1260)
        at org.apache.tools.ant.Main.runBuild(Main.java:853)
        at org.apache.tools.ant.Main.startAnt(Main.java:235)
        at org.apache.tools.ant.launch.Launcher.run(Launcher.java:285)
        at org.apache.tools.ant.launch.Launcher.main(Launcher.java:112)
Caused by: org.jruby.exceptions.RaiseException: (NameError) asciidoctor: FAILED: /home/rocky/tabletest/doc/bibtext.adoc: Failed to load AsciiDoc document - TreeProcessor not found in packages org.asciidoctor.extension; last error: cannot load Java class org.asciidoctor.extension.TreeProcessor
        at RUBY.const_missing(uri:classloader:/jruby/java/core_ext/module.rb:45)
        at org.jruby.RubyModule.const_get(org/jruby/RubyModule.java:3293)
        at RUBY.add_document_processor(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor/extensions.rb:1237)
        at RUBY.tree_processor(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor/extensions.rb:756)
        at RUBY.block in (root)(/var/lib/gems/2.3.0/gems/asciidoctor-bibtex-0.3.1/lib/asciidoctor-bibtex/asciidoctor.rb:6)
        at org.jruby.RubyBasicObject.instance_exec(org/jruby/RubyBasicObject.java:1724)
        at RUBY.block in activate(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor/extensions.rb:657)
        at org.jruby.RubyArray.each(org/jruby/RubyArray.java:1734)
        at RUBY.activate(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor/extensions.rb:652)
        at RUBY.initialize(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor/document.rb:460)
        at RUBY.load(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor.rb:1340)
        at RUBY.convert(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor.rb:1456)
        at RUBY.block in convert_file(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor.rb:1572)
        at org.jruby.RubyIO.open(org/jruby/RubyIO.java:1139)
        at RUBY.convert_file(/var/lib/gems/2.3.0/gems/asciidoctor-1.5.6/lib/asciidoctor.rb:1572)
        at RUBY.convertFile(<script>:68)

build.xml is:

<project xmlns:asciidoctor="antlib:org.asciidoctor.ant">

    <taskdef
      uri="antlib:org.asciidoctor.ant"
      resource="org/asciidoctor/ant/antlib.xml"
      classpath="lib/asciidoctor-ant-1.6.0-alpha.3.jar"/>

    <target name="withextension" depends="init">
        <asciidoctor:convert
          sourceDirectory="doc"
          sourceDocumentName="bibtext.adoc"
          outputDirectory="build"
          backend="html"
          gemPaths="/var/lib/gems/2.3.0:/home/rocky/.gem/ruby/2.3.0:/usr/lib/i386-linux-gnu/rubygems-integration/2.3.0:/usr/share/rubygems-integration/2.3.0:/usr/share/rubygems-integration/all">
        <require name="asciidoctor-bibtex"/>
      </asciidoctor:convert>
    </target>

</project>

The gempath is copied from the output of >gem environment

Using:
asciidoctor-bibtex version v0.3.1
Apache Ant(TM) version 1.9.6 compiled on July 8 2015
Java version: 1.8 in: /usr/lib/jvm/java-8-openjdk-i386/jre
asciidoctor-ant-1.6.0-alpha.3.jar
ruby 2.3.1p112 (2016-04-26) [i386-linux-gnu]

I assume everything is installed properly because this command works perfectly:

>asciidoctor -r asciidoctor-bibtex bibtext.adoc

Any help appreciated, especially suggestions for what to try next (I am a Ruby newbie).

Thanks