Downgrade celluloid to 0.16.0 because 0.16.1 was yanked.
Downgrade celluloid to 0.16.0 because 0.16.1 was yanked.

ruby-rcade/RubyGameDev.com,RailsGEnthusiasts/RubyGameDev.com,RailsGEnthusiasts/RubyGameDev.com,ruby-rcade/RubyGameDev.com,ruby-rcade/RubyGameDev.com,PowerCodeGirls/RubyGameDev.com,PowerCodeGirls/RubyGameDev.com,PowerCodeGirls/RubyGameDev.com,RailsGEnthusiasts/RubyGameDev.com