Fix for issue #4452 : undefined stdout in communicator.rb

Fixes #4452.
The stdout variable was changed during commit
https://github.com/mitchellh/vagrant/commit/138aa5aad375ac0c076a0e0a3493
e25906ce0434 . This commit just corrects this omission.
Merge pull request #4454 from npvisual/master

Fix for issue #4452 : undefined stdout in communicator.rb