Both sentences have the same meaning
I agree with David that both sentences end up meaning the same thing.
I perceive (but cannot assert) a slight difference in focus between both sentences:
- People hadn't travelled easily round the world from west to east until they dug the Suez Canal.
In this case, it seems to me that the focus lies on describing the situation before the construction of the canal: traveling from west to east was difficult and stopped being so when they constructed the canal.
- People didn't travel easily round the world from west to east until they had dug the Suez Canal.
In this case, it seems to me that, by using the negative, the focus lies on the situation after the construction of the canal: traveling started to be easy after they constructed the canal (once they had dug it, traveling became easier).