Update rspec requirement from ~> 3.8.0 to ~> 3.9.0

Updates the requirements on [rspec](https://github.com/rspec/rspec) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec/releases)
- [Commits](https://github.com/rspec/rspec/compare/v3.8.0...v3.9.0)

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

mb_string.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake", "~> 13.0.0"
-  spec.add_development_dependency "rspec", "~> 3.8.0"
+  spec.add_development_dependency "rspec", "~> 3.9.0"
   spec.add_development_dependency "simplecov", "~> 0.12.0"
 
   spec.add_development_dependency "coveralls", "~> 0.8.19"