Enable RSpec zero monkey patching mode

The plan for RSpec 4.0 is to disable monkey patching.

See https://github.com/rspec/rspec-core/blob/main/features/configuration/zero_monkey_patching_mode.feature for details.

spec/controllers/groups_controller_spec.rb

@@ -27,7 +27,7 @@
 #++
 require 'spec_helper'
 
-describe GroupsController do
+RSpec.describe GroupsController do
   let(:group) { create(:group, members: group_members) }
   let(:group_members) { [] }