Fix autoload error on Rails 5

It seems that only Rails 3.x needs these lines for the specs to run.

Author: bquorning

example_app_generator/generate_action_mailer_specs.rb

@@ -25,7 +25,7 @@ def comment_lines(path, flag, *args)
       end
     end
 
-    if defined?(ActionMailer)
+    if defined?(ActionMailer) && Rails::VERSION::MAJOR < 4
       # This will force the loading of ActionMailer settings
       ActionMailer::Base.smtp_settings
     end

example_app_generator/spec/support/default_preview_path

@@ -32,7 +32,7 @@ require_file_stub 'config/environment' do
     module ExampleApp
       class Application < Rails::Application
         config.eager_load = false
-        config.eager_load_paths.clear
+        config.eager_load_paths.clear if Rails::VERSION::MAJOR < 4
 
         # Don't care if the mailer can't send.
         config.action_mailer.raise_delivery_errors = false unless ENV['NO_ACTION_MAILER']