Fix "Don't know how to build task 'rails_env'" when running rake tasks

With Rails 4, running rake tasks such as "groonga:setup" is failing. This is because 'rails_env' task has been removed in Rails 4 (rails/rails@9abf589051006b62a2c7f8cf614eff629a083fd5).

This PR would fix the failure by first checking if "rails_env" task exists and then invoking the task in "groonga:load_config".