use "rake/clean" module now that it isn' so noisy; clean .yardoc folder

Rakefile.rb

@@ -8,6 +8,10 @@ end
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 require "yard"
+require "rake/clean"
+
+CLEAN.include %w[build_test_run .yardoc doc coverage]
+CLOBBER.include %w[pkg]
 
 RSpec::Core::RakeTask.new(:spec)
 
@@ -16,11 +20,3 @@ YARD::Rake::YardocTask.new do |yard|
 end
 
 task :default => :spec
-
-task :clean do
-  FileUtils.rm_rf(["build_test_run", "doc", "coverage"])
-end
-
-task :clobber => :clean do
-  FileUtils.rm_rf(["pkg"])
-end