Schema version number isn't part of the .json schema files #282

Given that we are about to do the first substantive new BeerJSON release in five years since the 1.0.2 one, it would be good for us to start including version number in beer.json and the other .json files in the schema directory.

In dotBeer, we include the version number in the $id field. In some sense this is a natural place to put things, but it might make things slightly more complicated if we want the $id to refer to a real URL for the latest version of the schema. (We side-step this in dotBeer by saying we don't guarantee $id is a real URL, as the JSON Schema standard does not require this.)

One alternative would be to add an x-version field.