Bump rdoc from 6.15.1 to 6.16.0

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.15.1 to 6.16.0.
- [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/v6.15.1...v6.16.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-version: 6.16.0
  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', '~> 6.15'
+  gem 'rdoc', '~> 6.16'
   gem 'browserstack-local', '~> 1.4'
 
   gem 'irb', '~> 1.15'