proto/example: Add version and fill in process

[wking]

I thought it would be easier to judge JSON compatibility if I filled
in a few more fields. The version addition is pretty minor, but I'm
having trouble getting the process fields to serialize. With
7831505, I get:

$ make
go run ./example.go
{
  "spec": {
    "version": "0.1.0",
    "platform": {
      "os": "linux",
      "arch": "x86_64"
    },
    "process": {
      "terminal": true,
      "user": {
        "type": 1
      },
      "args": [
        "sh"
      ],
      "env": [
        "TERM=linux"
      ],
      "cwd": "/root"
    }
  }
}

The extension fields aren't serialized, and I expect that has
something to do with:

$ grep -B4 -A2 'extension.*json' go/config.pb.go 
type User struct {
        // Type so that receivers of this message can `switch` for the fields
        // expected
        Type             *PlatformType             `protobuf:"varint,1,opt,name=type,enum=oci.PlatformType" json:"type,omitempty"`
        XXX_extensions   map[int32]proto.Extension `json:"-"`
        XXX_unrecognized []byte                    `json:"-"`
}

I'll take a stab at the protobuf3 branch and see if I have better luck
there.

Feel free to pull these commits into your protobuf branch or not as
you see fit.