delayed jobがエラーを吐くようになったので調べてみたら
delayed_job doesn't work (rails 3.1.3)
http://stackoverflow.com/questions/8932240/delayed-job-doesnt-work-rails-3-1-3
stackoverflowで解決。
collectiveidea / delayed_job
https://github.com/collectiveidea/delayed_job
gituhubに書いてあるとおりGemifileに書いて
Gemfile
gem 'delayed_job_active_record'
console
bundle install
で解決!