Log Message:
Update ruby-thor to 0.20.0.

0.20.0							2017/08/16

* Add check_default_type! to check if the default value of an option matches
  the defined type. It removes the warning on usage and gives the command
  authors the possibility to check for programming errors.

* Add disable_required_check! to disable check for required options in some
  commands. It is a substitute of disable_class_options that was not working
  as intended.

* Add inject_into_module.