Add rubocop-rake & fix Rake/Desc violations

Previously I was seeing the following output when running rubocop:

    Tip: Based on detected gems, the following RuboCop extension
    libraries might be helpful:
      * rubocop-rake (https://rubygems.org/gems/rubocop-rake)

    You can opt out of this message by adding the following to your
    config (see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions
    for more options):
      AllCops:
        SuggestExtensions: false

.rubocop.yml

@@ -1,3 +1,6 @@
+require:
+  - rubocop-rake
+
 inherit_from: .rubocop_todo.yml
 
 inherit_mode:

Rakefile

@@ -82,6 +82,7 @@ namespace 'test' do # rubocop:disable Metrics/BlockLength
   end
 end
 
+desc 'Run linters'
 task 'lint' do
   if defined?(RuboCop::RakeTask)
     RuboCop::RakeTask.new
@@ -132,10 +133,12 @@ if ENV['MOCHA_GENERATE_DOCS']
     task.options = ['--title', "Mocha #{Mocha::VERSION}", '--fail-on-warning']
   end
 
+  desc 'Ensure custom domain remains in place for docs on GitHub Pages'
   task 'checkout_docs_cname' do
     `git checkout docs/CNAME`
   end
 
+  desc 'Ensure custom JavaScript files remain in place for docs on GitHub Pages'
   task 'checkout_docs_js' do
     `git checkout docs/js/app.js`
     `git checkout docs/js/jquery.js`

gemfiles/Gemfile.rubocop

@@ -4,3 +4,4 @@ gemspec path: '../'
 
 gem 'rake'
 gem 'rubocop'
+gem 'rubocop-rake'