Update rake requirement from ~> 12.0 to ~> 13.0

Updates the requirements on [rake](https://github.com/ruby/rake) to permit the latest version.
- [Release notes](https://github.com/ruby/rake/releases)
- [Changelog](https://github.com/ruby/rake/blob/master/History.rdoc)
- [Commits](ruby/rake@v12.0.0...v13.0.0)

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

Author: dependabot-preview[bot]

split-api.gemspec

@@ -22,5 +22,5 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency 'bundler'
   gem.add_development_dependency 'rspec',       '~> 3.8.0'
   gem.add_development_dependency 'rack-test',   '~> 1.1'
-  gem.add_development_dependency 'rake',        '~> 12.0'
+  gem.add_development_dependency 'rake',        '~> 13.0'
 end