fix(builder): delete the git repo on `apps:destroy`

fixes #499 

This PR relies on #1161 and deis/base#3
Should probably define `etcd_safe_mkdir` and use it here - nice for consistency, nice to eat the output, and bad if this thing fails (which it will if the directory already exists).

```
function etcd_safe_mkdir {
  etcdctl --no-sync -C $ETCD mkdir $1 >/dev/null 2>&1 || true
}
```