Bump rdoc from 6.14.2 to 6.15.0

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.14.2 to 6.15.0.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](https://github.com/ruby/rdoc/compare/v6.14.2...v6.15.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-version: 6.15.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -63,7 +63,7 @@ end
 group :test do
     gem 'test-unit-full', '~> 0.0.5'
     gem 'rspec', '~> 3.13'
-    gem 'rdoc', '~> 6.14'
+    gem 'rdoc', '~> 6.15'
     gem 'browserstack-local', '~> 1.4'
 
     gem 'irb', '~> 1.15'