This is now required by Mongoid 4.x - when you call `update_attributes! text: 'foobar'` it would complain with this:

```
  1) Delayed::Backend::Mongoid::Job behaves like a delayed_job backend yaml serialization reloads changed attributes
     Failure/Error: story = Story.create(:text => 'hello')
     Mongoid::Errors::UnknownAttribute:
       
       Problem:
         Attempted to set a value for 'text' which is not allowed on the model Story.
       Summary:
         Without including Mongoid::Attributes::Dynamic in your model and the attribute does not already exist in the attributes hash, attempting to call Story#text= for it is not allowed. This is also triggered by passing the attribute to any method that accepts an attributes hash, and is raised instead of getting a NoMethodError.
       Resolution:
         You can include Mongoid::Attributes::Dynamic if you expect to be writing values for undefined fields often.
     Shared Example Group: "a delayed_job backend" called from ./spec/delayed_job_mongoid_spec.rb:4
```