Bump rails to 5.2 and update rubocop

app/admin/admin_users.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ActiveAdmin.register AdminUser do
   permit_params :email, :password, :password_confirmation
 

app/admin/attendees.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ActiveAdmin.register Attendee do
   scope :child?
   scope :diet?

app/admin/dashboard.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ActiveAdmin.register_page 'Dashboard' do
   menu priority: 1, label: proc { I18n.t('active_admin.dashboard') }
 

app/admin/guests.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ActiveAdmin.register Guest do
   permit_params :email, :first_name, :last_name, :attending, :diet, :songs,
                 :notes

app/admin/plus_ones.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ActiveAdmin.register PlusOne do
   permit_params :first_name, :last_name, :diet, :child