-Advantage in melee by beeing behind the target has been taken out due to feedback. Dispite it beeing a literal interpretation from the rules. (you get advantage if your target cant see you attacking)
You should only get advantage when you attack while being invisible or in stealth mode.
Attacking an enemy from behind when you are visible should not give advantage. Normally the enemy would see you and look at you when you attack him while you are visible.
I just checked the PHB and it does not mention flanking, so I am not sure about this.
Prof. Dr. Dr. Mad S. Tist 
