Update epubcheck-ruby requirement from ~> 5.0.0.0 to ~> 5.1.0.0 (#442)

Updates the requirements on [epubcheck-ruby](https://github.com/takahashim/epubcheck-ruby) to permit the latest version.
- [Commits](takahashim/epubcheck-ruby@v5.0.0.0...v5.1.0.0)

---
updated-dependencies:
- dependency-name: epubcheck-ruby
  dependency-type: direct:development
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -13,7 +13,7 @@ end
 
 group :optional do
   # epubcheck-ruby might be safe to be converted into runtime dependency, but could have issues when packaged into asciidoctorj-epub3
-  gem 'epubcheck-ruby', '~> 5.0.0.0'
+  gem 'epubcheck-ruby', '~> 5.1.0.0'
   # Kindlegen is unavailable neither for 64-bit MacOS nor for ARM
   gem 'kindlegen', '~> 3.1.0' unless RbConfig::CONFIG['host_os'] =~ /darwin/
 end