Support zeitwerk

[sanak]

Supports #4.

Changes proposed in this pull request:

• Support both Redmine 5.0 (zeitwerk) and Redmine 4.x
• Fix test error
• TODO: Remove the following warnings

  /path/to/redmine/plugins/redmine_supply/lib/redmine_resource_manager/save_resource_category.rb:7: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
  /path/to/redmine/plugins/redmine_supply/lib/redmine_resource_manager/save_resource_category.rb:10: warning: The called method `initialize' is defined here
  ../path/to/redmine/plugins/redmine_supply/lib/redmine_supply/action.rb:4: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
  /path/to/redmine/plugins/redmine_supply/lib/redmine_supply/save_supply_item.rb:6: warning: The called method `initialize' is defined here
  /path/to/redmine/plugins/redmine_supply/lib/redmine_resource_manager/save_resource_item.rb:7: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
  /path/to/redmine/plugins/redmine_supply/lib/redmine_resource_manager/save_resource_item.rb:10: warning: The called method `initialize' is defined here
  .................../path/to/redmine/plugins/redmine_supply/lib/redmine_supply/action.rb:4: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
  /path/to/redmine/plugins/redmine_supply/lib/redmine_supply/update_stock.rb:6: warning: The called method `initialize' is defined here

@gtt-project/maintainer

[sanak]

I merge this by myself.