Breaking change.
Going forward, PT's default serializer (PaperTrail::Serializers::YAML)
will use `safe_load` unless `ActiveRecord.use_yaml_unsafe_load`.
PT users are required to configure `ActiveRecord.yaml_column_permitted_classes`
correctly for their own application. Users may want to start with the following
safe-list:
```ruby
::ActiveRecord.use_yaml_unsafe_load = false
::ActiveRecord.yaml_column_permitted_classes = [
::ActiveRecord::Type::Time::Value,
::ActiveSupport::TimeWithZone,
::ActiveSupport::TimeZone,
::BigDecimal,
::Date,
::Symbol,
::Time
]
```
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