fix(publisher): do not publish older app versions

As soon as a newer version of an application is published to etcd,
we should not be publishing any older application versions. This
prevents containers which are orphaned by fleet or systemd to be
exposed by the router.

Manually tested and confirmed:

```
core@deis-2 ~ $ etcdctl ls /deis/services/pssample --recursive
/deis/services/pssample/pssample_v2.web.1
/deis/services/pssample/pssample_v2.web.2
core@deis-2 ~ $ etcdctl get /deis/services/pssample/pssample_v2.web.1
10.132.229.10:49238
core@deis-2 ~ $ etcdctl set /deis/services/pssample/pssample_v4.web.1 "10.132.229.10:49238"
10.132.229.10:49238
core@deis-2 ~ $ sleep 20 # wait for old keys to expire
core@deis-2 ~ $ etcdctl ls /deis/services/pssample --recursive
/deis/services/pssample/pssample_v4.web.1
```

fixes #1149
we error out significantly sooner at boot time. See main.go. client will only ever be `nil` with tests