Update epubcheck-ruby requirement from 5.0.0.0 to 5.1.0.0 (#351)

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:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Oscar Santiago <[email protected]>

Gemfile

@@ -13,6 +13,6 @@ gem 'asciidoctor-pdf', '2.3.7'
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.4.0'
 gem 'thread_safe', '0.3.6'
-gem 'epubcheck-ruby', '5.0.0.0'
+gem 'epubcheck-ruby', '5.1.0.0'
 gem 'html-proofer', '5.0.8'
 gem 'kindlegen', '3.1.1'