Fixed compatiblity with Mongoid 4.x and DelayedJob 4.x.

If you want to test this with Mongoid 4.x, and since neither Mongoid 4.x nor delayed_job_mongoid compatible with Mongoid 4.x has been released, you need to change Gemfile:

```
gem "mongoid", github: "mongoid/mongoid"
gem "activesupport", "~> 4.0"
gem "delayed_job", "~> 4.0"
gem "delayed_job_mongoid", github: "dblock/delayed_job_mongoid", branch: "mongoid-4x"
gem "actionmailer"
gem "shoulda"
gem "rake"
gem "rspec"
gem "ruby-debug19"
```

Corresponding change PRed into delayed_job_mongoid in https://github.com/collectiveidea/delayed_job_mongoid/pull/43.