Fixed compatibility with Mongoid 4.0.0.alpha2

fix for errors like that:

```
I, [2014-02-01T03:52:53.378349 #23936]  INFO -- : 2014-02-01T03:52:53-0500: [Worker(delayed_job host:some_server_name_here pid:23936)] Error while reserving job: The operation: #<Moped::Protocol::Command
  @length=436
  @request_id=8
  @response_to=0
  @op_code=2004
  @flags=[]
  @full_collection_name="collection_name_here.$cmd"
  @skip=0
  @limit=-1
  @selector={:findAndModify=>"delayed_backend_mongoid_jobs", :query=>{"run_at"=>{"$lte"=>2014-02-01 08:52:53 UTC}, "failed_at"=>nil, "$or"=>[{"locked_by"=>"delayed_job host:some_server_name_here pid:23936"}, {"locked_at"=>nil}, {"locked_at"=>{"$lt"=>2014-02-01 04:52:53 UTC}}]}, :new=>true, :sort=>{"locked_by"=>-1, "priority"=>1, "run_at"=>1}, :update=>{"$set"=>{:locked_at=>2014-02-01 08:52:53 UTC, :locked_by=>"delayed_job host:some_server_name_here pid:23936"}}}
  @fields=nil>
failed with error "unauthorized"
```