Use load hook for minitest integration

Unlike RSpec, Minitest doesn't load the Spec DSL when loaded by Bundler,
so loading the integration in the Railtie can be problematic.
Switch to a load hook so that the integration can be loaded whenever
minitest-rails is loaded, which it may not be during Draper initialization.

lib/draper/railtie.rb

@@ -20,7 +20,6 @@ class Railtie < Rails::Railtie
       if Rails.env.test?
         require 'draper/test_case'
         require 'draper/test/rspec_integration' if defined?(RSpec) and RSpec.respond_to?(:configure)
-        require 'draper/test/minitest_integration' if defined?(MiniTest::Rails)
       end
     end
 
@@ -50,6 +49,12 @@ class Railtie < Rails::Railtie
       end
     end
 
+    initializer "draper.minitest-rails_integration" do |app|
+      ActiveSupport.on_load :minitest do
+        require "draper/test/minitest_integration"
+      end
+    end
+
     console do
       require 'action_controller/test_case'
       ApplicationController.new.view_context