dep: move dev dependencies into Gemfile

and make Rubocop conditional on Ruby version, because 2.5 and 2.6 are
too old for the rubocop version we have, and it's causing CI to fail.

.rubocop.yml

@@ -22,3 +22,6 @@ Naming/InclusiveLanguage:
 Performance/CollectionLiteralInLoop:
   Exclude:
     - test/**/*.rb
+
+Rake/DuplicateTask:
+  Enabled: false

Gemfile

@@ -1,4 +1,21 @@
 # frozen_string_literal: true
 
 source "https://rubygems.org/"
+
 gemspec
+
+group :development do
+  gem("hoe-markdown", ["~> 1.3"])
+  gem("json", ["~> 2.2"])
+  gem("minitest", ["~> 5.14"])
+  gem("rake", ["~> 13.0"])
+  gem("rdoc", [">= 4.0", "< 7"])
+
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.0.0")
+    gem("rubocop", "~> 1.1")
+    gem("rubocop-minitest", "0.29.0")
+    gem("rubocop-performance", "1.16.0")
+    gem("rubocop-rake", "0.6.0")
+    gem("rubocop-shopify", "2.12.0")
+  end
+end

Rakefile

@@ -14,38 +14,44 @@ task :generate_safelists do
   load "tasks/generate-safelists"
 end
 
-require "rubocop/rake_task"
-
-module RubocopHelper
-  class << self
-    def common_options(task)
-      task.patterns += [
-        "Gemfile",
-        "Rakefile",
-        "lib",
-        "loofah.gemspec",
-        "test",
-      ]
+begin
+  require "rubocop/rake_task"
+
+  module RubocopHelper
+    class << self
+      def common_options(task)
+        task.patterns += [
+          "Gemfile",
+          "Rakefile",
+          "lib",
+          "loofah.gemspec",
+          "test",
+        ]
+      end
     end
   end
-end
 
-RuboCop::RakeTask.new do |task|
-  RubocopHelper.common_options(task)
-end
+  RuboCop::RakeTask.new do |task|
+    RubocopHelper.common_options(task)
+  end
 
-desc("Generate the rubocop todo list")
-RuboCop::RakeTask.new("rubocop:todo") do |task|
-  RubocopHelper.common_options(task)
-  task.options << "--auto-gen-config"
-  task.options << "--exclude-limit=50"
-end
-Rake::Task["rubocop:todo:autocorrect"].clear
-Rake::Task["rubocop:todo:autocorrect_all"].clear
+  desc("Generate the rubocop todo list")
+  RuboCop::RakeTask.new("rubocop:todo") do |task|
+    RubocopHelper.common_options(task)
+    task.options << "--auto-gen-config"
+    task.options << "--exclude-limit=50"
+  end
+  Rake::Task["rubocop:todo:autocorrect"].clear
+  Rake::Task["rubocop:todo:autocorrect_all"].clear
 
-task default: [:rubocop, :test]
+  task(default: :rubocop)
+rescue LoadError
+  warn("NOTE: rubocop not available")
+end
 
 desc "Print out the files packaged in the gem"
 task :debug_manifest do
   puts Bundler.load_gemspec("loofah.gemspec").files
 end
+
+task(default: :test)

loofah.gemspec

@@ -44,15 +44,4 @@ Gem::Specification.new do |spec|
 
   spec.add_runtime_dependency("crass", ["~> 1.0.2"])
   spec.add_runtime_dependency("nokogiri", [">= 1.5.9"])
-
-  spec.add_development_dependency("hoe-markdown", ["~> 1.3"])
-  spec.add_development_dependency("json", ["~> 2.2"])
-  spec.add_development_dependency("minitest", ["~> 5.14"])
-  spec.add_development_dependency("rake", ["~> 13.0"])
-  spec.add_development_dependency("rdoc", [">= 4.0", "< 7"])
-  spec.add_development_dependency("rubocop", "~> 1.1")
-  spec.add_development_dependency("rubocop-minitest", "0.29.0")
-  spec.add_development_dependency("rubocop-performance", "1.16.0")
-  spec.add_development_dependency("rubocop-rake", "0.6.0")
-  spec.add_development_dependency("rubocop-shopify", "2.12.0")
 end