Gemfile in asciidoctor-epub3-1.5.0.alpha.9 vs Gemfile in asciidoctor-epub3-1.5.0.alpha.10

@@ -1,15 +1,18 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 # Look in asciidoctor-epub3.gemspec for runtime and development dependencies.
 gemspec
 
+gem 'asciidoctor', ENV['ASCIIDOCTOR_VERSION'], require: false if ENV.key? 'ASCIIDOCTOR_VERSION'
+
 group :optional do
-  gem 'epubcheck', '3.0.1'
-  if (ruby_version = Gem::Version.new RUBY_VERSION) < (Gem::Version.new '2.0.0')
-    gem 'kindlegen', '2.9.4'
-    gem 'pygments.rb', '0.6.3'
-  else
-    gem 'kindlegen', '3.0.3'
-    gem 'pygments.rb', '1.1.2'
-  end
+  gem 'epubcheck-ruby', '4.1.1.0'
+  gem 'kindlegen', (Gem::Version.new RUBY_VERSION) < (Gem::Version.new '2.4.0') ? '3.0.3' : '3.0.5'
+  gem 'pygments.rb', '1.2.1'
+end
+
+group :docs do
+  gem 'yard', require: false
 end