Add rails-controller-testing gem

```
Spree::Admin::PaymentMethodsController#create and #update can create a payment method of a valid type
      Failure/Error: expect(response).to redirect_to spree.edit_admin_payment_method_path(assigns(:payment_method))

      NoMethodError:
        assigns has been extracted to a gem. To continue using it,
                add `gem 'rails-controller-testing'` to your Gemfile.
      # ./spec/controllers/spree/admin/payment_methods_controller_spec.rb:41:in `block (3 levels) in <module:Spree>'
```

Gemfile

@@ -168,6 +168,7 @@ group :test do
   gem 'simplecov', require: false
   gem 'test-prof'
   gem 'webmock'
+  gem 'rails-controller-testing'
   # See spec/spec_helper.rb for instructions
   # gem 'perftools.rb'
 end