fix(coreos-install): fix coreos-install URLs

deedubs in #coreos reported a problem with coreos-install:

```
./coreos-install -d /dev/disk/by-label/DOROOT
grep: /etc/lsb-release: No such file or directory
Downloading and verifying coreos_production_image.bin.bz2...
gpg: Signature made Tue 14 Jan 2014 07:23:12 AM UTC using RSA key ID 74E7E361
gpg: key 93D2DCB4 marked as ultimately trusted
gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: Good signature from "CoreOS Buildbot (Offical Builds) <buildbot@coreos.com>"
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  164M  100  164M    0     0  3790k      0  0:00:44  0:00:44 --:--:-- 4910k
coreos_production_image.bin.bz2: FAILED
sha1sum: WARNING: 1 computed checksum did NOT match
```

I tried to reproduce and got a different bug:

```
$ sudo coreos-install -d /dev/sda
/usr/bin/coreos-install: Image signature unavailable:
http://storage.core-os.net/coreos/amd64-generic/231.0.0/coreos_production_image.bin.bz2.DIGESTS.asc
```

I believe what happened was there is an old digest file that deedubs
downloaded from our mirror. In the case of the 231.0.0 build this old
digest file isn't there so I got a 404 instead.