Answer:
Contrary to widely-held belief, this question is a matter of personal preference. If you want to add an s after the apostrophé, you're free to do so. If you'd prefer the apostrophé alone (at the end of a word ending in 's'), you can do it that way, too. Both choices are acceptable to the language at large. The only time you have to follow someone else's rule is if you are writing for an organization that follows specific rules of style, in which case you have to abide by those rules. I personally prefer to add the 's', because that better reflects how most people pronounce the words.