gemspec: bump rake version

hci-theme.gemspec

@@ -31,5 +31,5 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "jekyll", "~> 4.0"
 
   spec.add_development_dependency "bundler", "~> 2.1"
-  spec.add_development_dependency "rake", "~> 12.0"
+  spec.add_development_dependency "rake", "~> 13.0"
 end