Guard not updating in Vagrant

Guard in the completed railstutorial.org sample app
(GitHub - mhartl/sample_app_3rd_edition: The sample app for the 3rd edition of the Ruby on Rails Tutorial) should
automatically
run the tests as soon as I save a change in any of the files.

When I try out the app in Nitrous.io, Guard works as expected. When I
save
a change to any of the files in the app, Guard AUTOMATICALLY runs the
tests
again.

However, when I try out the app in my Vagrant setup, Guard does NOT
automatically run the tests. I have to press the Enter key to initiate
the
tests. Thus, there must be something in Vagrant that prevents Guard
from
automatically running the tests.

Exactly what do I need to do in order to make sure that Guard
automatically
runs the tests?