Log Message:
devel/ruby-mercenary: update to 0.4.0

Update ruby-mercenary to 0.4.0.

## 0.4.0 / 2020-01-18

### Major Enhancements

  * Drop Ruby 2.3 support

### Minor Enhancements

  * Remove parent command's flags from subcommand usage (#44)

### Development Fixes

  * Adopt Jekyll's rubocop config for consistency

### Documentation

  * fixes the readme (#52)