Update asciidoctor requirement from 2.0.12 to 2.0.15 (#151)

Updates the requirements on [asciidoctor](https://github.com/asciidoctor/asciidoctor) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor/blob/master/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor@v2.0.12...v2.0.15)

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

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

Author: dependabot[bot]

Gemfile

@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 gem 'rake'
-gem 'asciidoctor', '2.0.12'
+gem 'asciidoctor', '2.0.15'
 
 gem 'json'
 gem 'awesome_print'