Adjust shippable.yml so that it builds correctly

Introduces the solution suggested at https://github.com/docker/docker/issues/1724 to get around the failing update:

```
sudo dpkg-divert --local --rename /usr/bin/ischroot && sudo ln -sf /bin/true /usr/bin/ischroot
```

Also minor tweak to the install directive (no need to shift up a directory now).  

Sorry about the messy git history.