Guard not updating in Vagrant

Guard in the completed railstutorial.org sample app (https://github.com/mhartl/sample_app_3rd_edition.git) 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?