Before enabling hibernation, please try to test whether it works correctly by running pm-hibernate in a terminal. The system will try to hibernate. If you are able to start the system again then you are more or less safe to add an override.
To do so, start editing
sudo nano /etc/polkit-1/localauthority/50-local.d/com.ubuntu.enable-hibernate.pkla
Fill it with this
[Re-enable hibernate by default] Identity=unix-user:* Action=org.freedesktop.upower.hibernate ResultActive=yes
Save by pressing Ctrl-O and exit nano by pressing Ctrl-X
Restart and hibernation is back!