$ bundle exec spork
Using RSpec, Rails
Preloading Rails environment
Loading Spork.prefork block...
Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
対処法
言われた通りに、
require 'minitest/autorun'
をspec/spec_helper.rbに追加
$ spork
↑$ bundle exec sporkと同じ
Using RSpec, Rails
Preloading Rails environment
Loading Spork.prefork block...
Spork is ready and listening on 8989!
warning がなくなった^^!