Use require_relative() for "prototype runtime --require-relative"

[tadd]

As of now, rbs prototype runtime --require-relative is identical with ... --require because it uses require(), not require_relative().

This PR changes --require-relative to use require_relative().

With this change, the below command works as expected with local rails application.

bundle exec rbs prototype runtime --require-relative config/environment ModelName

(Note: I needed to add Rails.application.eager_load! to environment.rb for proper recognition of classes)

[soutaro]

👍