Use be_successful instead of be_success

We were getting a deprecation message in Rails 5.2:

> The success? predicate is deprecated and will be removed in Rails 6.0.
> Please use successful? as provided by Rack::Response::Helpers.

spec/controllers/legislation/processes_controller_spec.rb

@@ -8,6 +8,6 @@ describe Legislation::ProcessesController do
 
     get :summary, params: { id: legislation_process, format: :xlsx }
 
-    expect(response).to be_success
+    expect(response).to be_successful
   end
 end