Support ArchLinux bootstrap

Hi. I wrote ArchLinux support so I'm glad to have you guys reviews.

For the record:
- https://www.opscode.com/chef/install.sh doesn't support ArchLinux.
- ruby-chef and its dependees mostly belong to AUR so I decide to use yaourt.
- ruby-chef seems to have a lack of dependent "pry" so I patched it.