Display hostname with example app

This is a trivial addition to show the container ID (hostname) with the example app. It can be used to demonstrate application scaling:

```console
$ deis ps:scale web=3
Scaling processes... but first, coffee!
done in 7s
=== jiggly-umbrella Processes

--- web:
web.1 up (v4)
web.2 up (v4)
web.3 up (v4)
$ curl -s http://jiggly-umbrella.dev.rovid.io
Powered by Deis!
Running on container ID 7322d0ffbcac
$ curl -s http://jiggly-umbrella.dev.rovid.io
Powered by Deis!
Running on container ID 13e7e3c1c24d
$ curl -s http://jiggly-umbrella.dev.rovid.io
Powered by Deis!
Running on container ID eee0faf14518
```