Rely on rubocop-github for Ruby formatting

.rubocop.yml

@@ -1,6 +1,10 @@
+inherit_gem:
+  rubocop-github:
+    - config/default.yml
+
 AllCops:
   Exclude:
     - _site/**/*
 
-Metrics/LineLength:
+Layout/LineLength:
   Enabled: false

Gemfile

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
-source 'https://rubygems.org'
+source "https://rubygems.org"
 
 gemspec

jekyll-theme-primer.gemspec

@@ -1,24 +1,26 @@
 # frozen_string_literal: true
 
 Gem::Specification.new do |s|
-  s.name          = 'jekyll-theme-primer'
-  s.version       = '0.5.4'
-  s.authors       = ['GitHub, Inc.']
-  s.email         = ['[email protected]']
-  s.homepage      = 'https://github.com/pages-themes/jekyll-theme-primer'
+  s.name          = "jekyll-theme-primer"
+  s.version       = "0.5.4"
+  s.authors       = ["GitHub, Inc."]
+  s.email         = ["[email protected]"]
+  s.homepage      = "https://github.com/pages-themes/jekyll-theme-primer"
   s.summary       = "Primer is a Jekyll theme for GitHub Pages based on GitHub's Primer styles"
 
   s.files         = `git ls-files -z`.split("\x0").select do |f|
     f.match(%r{^(assets|_(includes|layouts|sass)/|(LICENSE|README)((\.(txt|md)|$)))}i)
   end
 
   s.platform      = Gem::Platform::RUBY
-  s.license       = 'MIT'
+  s.license       = "MIT"
 
-  s.add_dependency 'jekyll', '> 3.5', '< 5.0'
-  s.add_runtime_dependency 'jekyll-github-metadata', '~> 2.9'
-  s.add_runtime_dependency 'jekyll-seo-tag', '~> 2.0'
-  s.add_development_dependency 'html-proofer', '~> 3.0'
-  s.add_development_dependency 'rubocop', '~> 0.50'
-  s.add_development_dependency 'w3c_validators', '~> 1.3'
+  s.required_ruby_version = ">= 2.4.0"
+
+  s.add_dependency "jekyll", "> 3.5", "< 5.0"
+  s.add_runtime_dependency "jekyll-github-metadata", "~> 2.9"
+  s.add_runtime_dependency "jekyll-seo-tag", "~> 2.0"
+  s.add_development_dependency "html-proofer", "~> 3.0"
+  s.add_development_dependency "rubocop-github", "~> 0.16"
+  s.add_development_dependency "w3c_validators", "~> 1.3"
 end

script/validate-html

@@ -1,13 +1,13 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
-require 'w3c_validators'
+require "w3c_validators"
 
 def validator(file)
   extension = File.extname(file)
-  if extension == '.html'
+  if extension == ".html"
     W3CValidators::NuValidator.new
-  elsif extension == '.css'
+  elsif extension == ".css"
     W3CValidators::CSSValidator.new
   end
 end
@@ -18,10 +18,10 @@ def validate(file)
   path = File.expand_path "../_site/#{file}", __dir__
   results = validator(file).validate_file(path)
 
-  return puts 'Valid!' if results.errors.empty?
+  return puts "Valid!" if results.errors.empty?
 
   results.errors.each { |err| puts err.to_s }
   exit 1
 end
 
-validate 'index.html'
+validate "index.html"