DISCUSS: Activity versioning clarification

Took a stab at resolving: https://github.com/adlnet/xAPI-Spec/issues/29 , but I'm really not sure we need to list this non-requirement here.

It did come up as a point of confusion in Alexandria (resulting in this issue), so it may be worth including.
updated based on feedback

fixed typo, changed to "no behavioral implications" as suggested by @fugu13
Merge pull request #269 from bscSCORM/actversion

DISCUSS: Activity versioning clarification