Upgrade to Rake 13 to fix master (#381)

Merge pull request 381

Author: jez

jekyll-feed.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "nokogiri", "~> 1.6"
-  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-jekyll", "~> 0.12.0"
   spec.add_development_dependency "typhoeus", ">= 0.7", "< 2.0"