Update json requirement from 2.4.1 to 2.5.1

Updates the requirements on [json](https://github.com/flori/json) to permit the latest version.
- [Release notes](https://github.com/flori/json/releases)
- [Changelog](https://github.com/flori/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.4.1...v2.5.1)

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

Author: dependabot[bot]

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gem 'rake', ' 13.0.1'
 gem 'asciidoctor', '2.0.12'
 
-gem 'json', '2.4.1'
+gem 'json', '2.5.1'
 gem 'awesome_print', '1.8.0'
 
 gem 'asciidoctor-epub3', '1.5.0.alpha.19'