debian: Remove ‘-y’ from stanford-keyring install

manifests/os/debian.pp

@@ -220,7 +220,6 @@ class base::os::debian::apt (
   # Install the stanford-keyring package
   package { 'stanford-keyring':
     require         => File['/etc/apt/sources.list.d/stanford.list'],
-    install_options => [ '-y' ],
     notify          => Exec['aptitude update'],
   }
 }