"can't be called from trap context" shutdown issue on Ruby 2.0.0

[jmettraux]

Initially reported at: http://stackoverflow.com/questions/21733308/gracefully-stop-rufus-scheduler-on-heroku-worker

test code: https://gist.github.com/jmettraux/a4c00374f58e9f7affa8

output:

/home/jmettraux/w/rufus-scheduler/lib/rufus/scheduler/job_array.rb:74:
  in `synchronize': can't be called from trap context (ThreadError)
    from /home/jmettraux/w/rufus-scheduler/lib/rufus/scheduler/job_array.rb:74:in `to_a'
    from /home/jmettraux/w/rufus-scheduler/lib/rufus/scheduler.rb:276:in `jobs'
    from /home/jmettraux/w/rufus-scheduler/lib/rufus/scheduler.rb:127:in `shutdown'
    from t.rb:8:in `block in <main>'
    from t.rb:18:in `call'
    from t.rb:18:in `sleep'
    from t.rb:18:in `<main>'

https://www.ruby-forum.com/topic/4411227