:js => true
tells Capybara to use the JavaScript driver for your spec; it's needed to run pages that have to execute JavaScript as part of their functionality. In your case, it would be needed for the JS confirmation dialog.
BUT...there's a couple of other things to note:
- Your link is titled "Account" above and your looking for "Account Settings". Is that a typo?
- I don't think you'll be able to interact with a vanilla browser confirmation dialog (i.e. alert box with confirm buttons). I know you can't when using the Poltergeist driver