As somebody who is also thinking about a career break and traveling i would say - be selfish and don't tell your boss. You don't stand to gain anything but lose everything by being honest!
They are a business, not a friend or family so it's not like you are dropping a mate in it.
.......Unless after you finish traveling you intend to go back to this place and your area of employment is particularly hard to get back into?
Even then i would still say be selfish and put yourself first.