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/projects_controller_spec.rb

@@ -28,7 +28,7 @@
 
 require 'spec_helper'
 
-describe ProjectsController do
+RSpec.describe ProjectsController do
   shared_let(:admin) { create(:admin) }
   let(:non_member) { create(:non_member) }