simplified service update message to user (#587)
simplified service update message to user (#587)

```bash
➜  client git:(master) ✗ ./kn service update summary-fn --untag sync1
Using kn config file: /Users/maximilien/.kn/config.yaml
Updating Service 'summary-fn' in namespace 'default':

  0.089s Ready to serve.

Service 'summary-fn' with latest revision 'summary-fn-nxqwb-41' (unchanged) is available at URL:
http://summary-fn-default.kndemo-267179.sjc03.containers.appdomain.cloud

➜  client git:(master) ✗ ./kn service update summary-fn --image docker.io/drmax/summary-fn:latest
Using kn config file: /Users/maximilien/.kn/config.yaml
Updating Service 'summary-fn' in namespace 'default':

  0.091s The Configuration is still working to reflect the latest desired specification.
  6.317s Ready to serve.

Service 'summary-fn' updated to latest revision 'summary-fn-jfjcy-42' is available at URL:
http://summary-fn-default.kndemo-267179.sjc03.containers.appdomain.cloud
```
knative/client,knative/client