Merge pull request #1940 from progit/dependabot/bundler/asciidoctor-pdf-2.3.15

Update asciidoctor-pdf requirement from 2.3.14 to 2.3.15

Author: ben

Gemfile

@@ -8,7 +8,7 @@ gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-fb2', '0.7.0'
 gem 'asciidoctor-epub3', '2.1.0'
-gem 'asciidoctor-pdf', '2.3.14'
+gem 'asciidoctor-pdf', '2.3.15'
 
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.4.1'