Update asciidoctor-epub3 requirement

Updates the requirements on [asciidoctor-epub3](https://github.com/asciidoctor/asciidoctor-epub3) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-epub3/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-epub3/blob/master/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor-epub3@v1.5.0.alpha.18...v1.5.0.alpha.19)

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

Author: dependabot[bot]

Gemfile

@@ -6,7 +6,7 @@ gem 'asciidoctor', '2.0.11'
 gem 'json', '2.3.1'
 gem 'awesome_print', '1.8.0'
 
-gem 'asciidoctor-epub3', '1.5.0.alpha.18'
+gem 'asciidoctor-epub3', '1.5.0.alpha.19'
 gem 'asciidoctor-pdf', '1.5.3'
 
 gem 'coderay', '1.1.3'