It's a question of individual style. Traditionally, the apostrophe has been used, but many modern writers and publishers prefer not to use the apostrophe.
There's no real reason for it, except that even rules of punctuation change over time. If you are working for a newspaper or a magazine, you need to follow the in-house style guide. If you are writing something for school or college, you need to follow whatever the teacher or tutor says is correct. If you are writing for yourself, it doesn't matter, as long as you are consistent.
- For Teachers