Support Asciidoctor PDF 2.3.x

[wilkinsona]

Trying to use the PDF backend with Asciidoctor PDF 2.3.7 results in the following failure:

Caused by: org.jruby.exceptions.NoMethodError: (NoMethodError) undefined method `keep_together' for #<SpringPdfConverter:0x26562718>
        at RUBY.convert_listing_or_literal(uri:classloader:/gems/spring-asciidoctor-backends-0.0.0/lib/spring-asciidoctor-backends/spring-pdf-converter.rb:38)
        at uri_3a_classloader_3a_.gems.asciidoctor_minus_pdf_minus_2_dot_3_dot_7.lib.asciidoctor.pdf.converter.convert(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:152)
        at RUBY.convert(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/abstract_block.rb:76)
        at RUBY.content(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/abstract_block.rb:85)
        at org.jruby.RubyArray.map(org/jruby/RubyArray.java:2809)
        at RUBY.content(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/abstract_block.rb:85)
        at RUBY.traverse(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:2387)
        at RUBY.convert_section(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:682)
        at uri_3a_classloader_3a_.gems.asciidoctor_minus_pdf_minus_2_dot_3_dot_7.lib.asciidoctor.pdf.converter.convert(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:152)
        at RUBY.convert(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/abstract_block.rb:76)
        at RUBY.content(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/abstract_block.rb:85)
        at org.jruby.RubyArray.map(org/jruby/RubyArray.java:2809)
        at RUBY.content(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/abstract_block.rb:85)
        at RUBY.content(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/document.rb:1012)
        at RUBY.traverse(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:2387)
        at RUBY.convert_document(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:292)
        at RUBY.indent_section(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:616)
        at RUBY.convert_document(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:200)
        at RUBY.convert(uri:classloader:/gems/asciidoctor-pdf-2.3.7/lib/asciidoctor/pdf/converter.rb:152)
        at RUBY.convert(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/document.rb:956)
        at RUBY.convert(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/convert.rb:118)
        at RUBY.convert_file(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/convert.rb:190)
        at org.jruby.RubyIO.open(org/jruby/RubyIO.java:1176)
        at RUBY.convert_file(uri:classloader:/gems/asciidoctor-2.0.20/lib/asciidoctor/convert.rb:190)