Update epubcheck-ruby requirement from 4.2.4.0 to 4.2.5.0 (#207)

Updates the requirements on [epubcheck-ruby](https://github.com/takahashim/epubcheck-ruby) to permit the latest version.
- [Release notes](https://github.com/takahashim/epubcheck-ruby/releases)
- [Commits](takahashim/epubcheck-ruby@v4.2.4.0...v4.2.5.0)

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

Gemfile

@@ -12,6 +12,6 @@ gem 'asciidoctor-pdf', '1.5.4'
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.2.0'
 gem 'thread_safe', '0.3.6'
-gem 'epubcheck-ruby', '4.2.4.0'
+gem 'epubcheck-ruby', '4.2.5.0'
 gem 'html-proofer', '3.17.3'