Replace script/delayed_job with bin/delayed_job for Rails 4 compatbility

I started a new Rails 4 app and realised the generator was creating RAILS_ROOT/script/delayed_job. This directory has been replaced in Rails 4 with RAILS_ROOT/bin and binstubs. I'm using this binstub in production.

I noticed there are no tests for the generators. I've tested it locally with a brand new Rails 4 app.

I'm also happy to write a new rake task for upgrading from DJ 3 to DJ 4.