Version the REST API via Accept header?
It seems like including the API version in the URL isn't generally viewed as best practice. Another option is to use the Accept header, e.g. with a version=3.1
parameter. OT1H versioning urls can break permalinks and may have other problems. OTOH, Accept headers are more difficult to test and describe.
Here's an interesting discussion on the topic: http://www.troyhunt.com/2014/02/your-api-versioning-is-wrong-which-is.html