Bump rails to 5.2 and update rubocop

test/models/admin_user_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class AdminUserTest < ActiveSupport::TestCase

test/models/attendee_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class AttendeeTest < ActiveSupport::TestCase

test/models/guest_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class GuestTest < ActiveSupport::TestCase

test/models/plus_one_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class PlusOneTest < ActiveSupport::TestCase