Update assertion on redis in generated Gemfile

Redis now included in Gemfile but commented out. This change was made in
91864439 and is causing the test
failure.

See https://travis-ci.org/rails/rails/jobs/106994913#L1025

railties/test/generators/app_generator_test.rb

@@ -415,7 +415,7 @@ def test_generator_if_skip_action_cable_is_given
 
   def test_action_cable_redis_gems
     run_generator
-    assert_gem 'redis'
+    assert_file "Gemfile", /^# gem 'redis'/
   end
 
   def test_inclusion_of_javascript_runtime