Use head instead of render: nothing in tests

`render nothing: true` was removed from Rails 5.1 and has no effect
anymore -- it was replaced with `head`.

spec/acceptance/rails_integration_spec.rb

@@ -26,7 +26,7 @@ describe 'shoulda-matchers integrates with Rails' do
       class ExamplesController < ApplicationController
         def show
           @example = 'hello'
-          render nothing: true
+          head :ok
         end
       end
     FILE

spec/unit/shoulda/matchers/action_controller/permit_matcher_spec.rb

@@ -586,7 +586,7 @@ describe Shoulda::Matchers::ActionController::PermitMatcher, type: :controller d
           end
         end
 
-        render nothing: true
+        head :ok
       end
     end