Fix canceling the add review modal on mobile (bug 1071588)

https://bugzilla.mozilla.org/show_bug.cgi?id=1071588
So, using `require('navigation').back();` works, but then I'm always redirected to the reviews listing page, even if I was coming from the apps detail page. I'm not familiar enough with the navigation in fireplace to figure out why, but that's not what we want. Any way to avoid that ?