Add where_object query to PaperTrail::Version.

I wasn't able to get this to work with native JSON types (an error will be raised when the query is attempted) due to not being able to accurately query for the full set of possible types (JSON's `null`, for instance). If anyone is able to give direction on how to make these queries work, please share!

I also wasn't sure if there's a better way to test this functionality; again, feedback welcome!