Enable RSpec/NotToNot cop and fix all issues

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NotToNot

spec/features/admin/users_spec.rb

@@ -21,8 +21,8 @@ feature 'Admin users' do
 
     expect(page).to have_content @user.name
     expect(page).to have_content @user.email
-    expect(page).to_not have_content @admin.name
-    expect(page).to_not have_content @admin.email
+    expect(page).not_to have_content @admin.name
+    expect(page).not_to have_content @admin.email
   end
 end