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>

pushmi_pullyu.gemspec

@@ -40,7 +40,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'danger', '~> 6.0'
   spec.add_development_dependency 'pry', '~> 0.10', '>= 0.10.4'
   spec.add_development_dependency 'pry-byebug', '~> 3.6'
-  spec.add_development_dependency 'rake', '~> 12.0'
+  spec.add_development_dependency 'rake', '~> 13.0'
   spec.add_development_dependency 'rspec', '~> 3.0'
   spec.add_development_dependency 'rubocop', '~> 0.51'
   spec.add_development_dependency 'rubocop-rspec', '~> 1.10'