appc/spec: make version a first-class required field

Hrm, I forgot to add this to the spec before we converted it to markdown:

"For example, given the app name `example.com/reduce-worker` and the required labels version=1.0.0,arch=amd64,os=linux we render a template like this:

https://example.com/reduce-worker-1.0.0-linux-amd64.aci

The template used is:

https://{name}-{version}-{os}-{arch}.aci"