Fix package management error during xenial, wily setup
Fix package management error during xenial, wily setup

"Error: Package gnome-session-fallback has no installation candidate" ― then a note that it has been replaced with gnome-session-flashback, so contributing a change to crouton to reflect this

dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton