Avoid scope error in tests (Mongoid 4)

Mongoid 4 throws the following error when running specs. This PR only modifies the specs to fix it. Verified it works on Mongoid 3 as well.

```
/lib/mongoid/scopable.rb:306:in `check_scope_validity':  (Mongoid::Errors::InvalidScope)
Problem:
  Defining a scope of value #<Mongoid::Slug::Criteria:0x4195c20> on Page is not allowed.
Summary:
  Scopes in Mongoid must be procs that wrap criteria objects.
Resolution:
  Change the scope to be a proc wrapped critera.

 Example:
   class Band
     include Mongoid::Document
     scope :inactive, ->{ where(active: false) }
   end
```