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](rspec/rspec@v3.8.0...v3.9.0)

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

Author: dependabot-preview[bot]

split-api.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'multi_json'
 
   gem.add_development_dependency 'bundler'
-  gem.add_development_dependency 'rspec',       '~> 3.8.0'
+  gem.add_development_dependency 'rspec',       '~> 3.9.0'
   gem.add_development_dependency 'rack-test',   '~> 1.1'
   gem.add_development_dependency 'rake',        '~> 13.0'
 end