Use native Rake directory

Rake has long automatically imported additional Rake tasks from the
rakelib directory. This commit renames the tasks directory to rakelib
and removes custom logic from the top-level Rakefile that imported Rake
tasks from the tasks directory.

Author: mhashizume

Rakefile

@@ -5,8 +5,6 @@ require 'open3'
 require 'rspec/core/rake_task'
 require 'facter/version'
 
-Dir.glob(File.join('tasks/**/*.rake')).each { |file| load file }
-
 task default: :spec
 
 desc 'Generate changelog'