Fix Rails 5.1 deprecation warnings
Fix Rails 5.1 deprecation warnings

```
DEPRECATION WARNING: Passing a class to the `class_name` is deprecated and will raise an ArgumentError in Rails 5.2. It eagerloads more classes than necessary and potentially creates circular dependencies. Please pass the class name as a string: `has_many :tracked_changes, class_name: 'Fae::Change'`
```

tshedor/fae,wearefine/fae,wearefine/fae,tshedor/fae,wearefine/fae,tshedor/fae