Update rake requirement from ~> 12 to ~> 13

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...v13.0.1)

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

Author: dependabot-preview[bot]

split.gemspec

@@ -36,7 +36,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'bundler',     '>= 1.17'
   s.add_development_dependency 'simplecov',   '~> 0.15'
   s.add_development_dependency 'rack-test',   '~> 1.1'
-  s.add_development_dependency 'rake',        '~> 12'
+  s.add_development_dependency 'rake',        '~> 13'
   s.add_development_dependency 'rspec',       '~> 3.7'
   s.add_development_dependency 'pry',         '~> 0.10'
   s.add_development_dependency 'fakeredis',   '~> 0.7'