Acceptance test: check if pip installation is idempotent

Second part of regression test for PR#418

spec/acceptance/virtualenv_spec.rb

@@ -53,6 +53,7 @@ class { 'python' :
 
       # Run it twice and test for idempotency
       apply_manifest(pp, catch_failures: true)
+      apply_manifest(pp, catch_changes: true)
     end
   end
 end