Switch to ubuntu-debootstrap:14.04

This saves another 100mb image size and some deis components are already using `ubuntu-debootstrap:14.04` so its likely already pulled.

```
$ docker images | grep example
example-after                             latest              5c5002266a68        5 seconds ago       87.16 MB
example-before                            latest              9025170bd3f0        6 minutes ago       192.7 MB
$ docker run -i -t example-after /bin/bash
root@c22cb636e444:/# 
```