Use MiniTest::Unit::TestCase instead of Minitest::Test

Since Rails 4.0.x is depending on Minitest 4.x, the constant won't be
available so the gist won't run.

[ci skip]

guides/bug_report_templates/active_record_gem.rb

@@ -25,7 +25,7 @@ class Comment < ActiveRecord::Base
   belongs_to :post
 end
 
-class BugTest < Minitest::Test
+class BugTest < MiniTest::Unit::TestCase
   def test_association_stuff
     post = Post.create!
     post.comments << Comment.create!