Mongoid 3 compatibility

Fixed the tests and made the gem work with mongoid < 4
You might want to consider not using moped to write object to db, but instead use simulate real race condition:

```ruby 
10.times do
  Thread.new(required_fields) do |required_fields|
    described_class.new(required_fields).save
  end
end

expect( described_class.count ).to eq 1
```

(Didn't test this but should work)