doc: fix layout
logger: require files explicitly

require_files_under_directory_in_load_path ignores exceptions while
loading. If window system can't be initialized, 'require "gtk2"' is
failed. But the exception is also ignored. Then rabbit command is
crashed on no window system environment such as "DISPLAY= rabbit".

[shocker-ja:1189]

Reported by Kazuhiro NISHIYAMA. Thanks!!!
Add missing require