Merge pull request #1929 from progit/dependabot/bundler/asciidoctor-epub3-2.1.0

Update asciidoctor-epub3 requirement from 1.5.1 to 2.1.0

Author: ben

Gemfile

@@ -7,7 +7,7 @@ gem 'json', '2.6.3'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-fb2', '0.7.0'
-gem 'asciidoctor-epub3', '1.5.1'
+gem 'asciidoctor-epub3', '2.1.0'
 gem 'asciidoctor-pdf', '2.3.12'
 
 gem 'coderay', '1.1.3'