Update epubcheck-ruby requirement from 4.2.5.0 to 4.2.6.0

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@v4.2.5.0...v4.2.6.0)

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

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

Author: dependabot[bot]

Gemfile

@@ -12,6 +12,6 @@ gem 'asciidoctor-pdf', '1.6.0'
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.2.0'
 gem 'thread_safe', '0.3.6'
-gem 'epubcheck-ruby', '4.2.5.0'
+gem 'epubcheck-ruby', '4.2.6.0'
 gem 'html-proofer', '3.19.2'
 gem 'kindlegen', '3.1.1'