Remove ActiveSupport::Concern

Quick summary.

- Removes use of `ActiveSupport::Concern` as it’s generally not actually needed (I think someone once quipped: “use of `ActiveSupport::Concern` is concerning” :wink:)

- Changes to extending `PgSearch` instead of including. Raises an error when trying to `include` to inform the user.

- Moves `pg_search_rank` to `PgSearch::Compatibility::ActiveRecord3` and only include if needed to indicate it’s not needed for ActiveRecord 4+. This is the lone instance method on PgSearch objects we add, and @nertzy agrees we may want to rethink adding it by default.

The first two points will make #148 easier as we will no longer have to delay the requiring of `PgSearch::Document` till after we add all methods to `PgSearch` (since `ActiveSupport::Concern` was the real problem causing that need).