README formatting [ci skip]
Update appraisals
Make `Paraphrase::ActiveModel` @api private
Fix `spec_helper.rb`

`ActiveRecord::Relation` does not have `Paraphrase::Syntax` included by
the railtie.
Refactor how source relation is determined

Limit `Query` to only work with `ActiveRecord::Relation` instances. Let
`ActiveRecord::Base` and `ActiveRecord::Relation` ensure this instead of
`Query`. Removes `source` DSL in favor of setting the class attribute
directly.