Update rspec requirement from ~> 3.11.0 to ~> 3.12.0

Updates the requirements on [rspec](https://github.com/rspec/rspec-metagem) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec-metagem/releases)
- [Commits](rspec/rspec-metagem@v3.11.0...v3.12.0)

---
updated-dependencies:
- dependency-name: rspec
  dependency-type: direct:development
...

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

Author: dependabot[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',       '~> 3.11.0'
+  gem.add_development_dependency 'rspec',       '~> 3.12.0'
   gem.add_development_dependency 'rack-test',   '~> 2.1'
   gem.add_development_dependency 'rake',        '~> 13.0'
 end