Update ubuntu with new Core tarballs and a dist-upgrade RUN line

```console
root@88627d7e5342:/stackbrew/stackbrew# ./brew-cli --debug --no-namespace -b ubuntu --targets ubuntu git://github.com/tianon/stackbrew
2014-08-11 14:55:43,314 DEBUG Cloning repo_url=git://github.com/tianon/stackbrew, ref=ubuntu
2014-08-11 14:55:43,315 DEBUG folder = /tmp/tmpqsPNqY
2014-08-11 14:55:43,315 DEBUG Cloning git://github.com/tianon/stackbrew into /tmp/tmpqsPNqY
2014-08-11 14:55:43,315 DEBUG Executing "git clone git://github.com/tianon/stackbrew ." in /tmp/tmpqsPNqY
Cloning into '.'...
remote: Counting objects: 1084, done.
remote: Compressing objects: 100% (476/476), done.
remote: Total 1084 (delta 603), reused 1070 (delta 596)
Receiving objects: 100% (1084/1084), 161.65 KiB | 215.00 KiB/s, done.
Resolving deltas: 100% (603/603), done.
Checking connectivity... done.
2014-08-11 14:55:44,540 DEBUG Checkout ref:ubuntu in /tmp/tmpqsPNqY
2014-08-11 14:55:44,540 DEBUG Executing "git checkout ubuntu" in /tmp/tmpqsPNqY
Branch ubuntu set up to track remote branch ubuntu from origin.
Switched to a new branch 'ubuntu'
2014-08-11 14:55:44,546 DEBUG 12.04.5: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 precise

2014-08-11 14:55:44,546 DEBUG 12.04: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 precise

2014-08-11 14:55:44,546 DEBUG precise: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 precise

2014-08-11 14:55:44,546 DEBUG 14.04.1: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 trusty

2014-08-11 14:55:44,546 DEBUG 14.04: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 trusty

2014-08-11 14:55:44,546 DEBUG trusty: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 trusty

2014-08-11 14:55:44,546 DEBUG latest: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 trusty

2014-08-11 14:55:44,546 DEBUG 14.10: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 utopic

2014-08-11 14:55:44,546 DEBUG utopic: git://github.com/tianon/docker-brew-ubuntu-core@772cc75547841c42b9eb71bcffa0904b2329d911 utopic

2014-08-11 14:55:44,546 DEBUG ubuntu: 12.04.5,12.04,precise
2014-08-11 14:55:44,546 DEBUG ubuntu: 14.10,utopic
2014-08-11 14:55:44,546 DEBUG ubuntu: 14.04.1,14.04,trusty,latest
2014-08-11 14:55:44,546 DEBUG Cloning repo_url=git://github.com/tianon/docker-brew-ubuntu-core, ref=772cc75547841c42b9eb71bcffa0904b2329d911
2014-08-11 14:55:44,547 DEBUG folder = /tmp/tmpk7D4HJ
2014-08-11 14:55:44,547 DEBUG Cloning git://github.com/tianon/docker-brew-ubuntu-core into /tmp/tmpk7D4HJ
2014-08-11 14:55:44,547 DEBUG Executing "git clone git://github.com/tianon/docker-brew-ubuntu-core ." in /tmp/tmpk7D4HJ
Cloning into '.'...
remote: Counting objects: 57, done.
remote: Compressing objects: 100% (48/48), done.
remote: Total 57 (delta 17), reused 16 (delta 6)
Receiving objects: 100% (57/57), 161.98 MiB | 3.40 MiB/s, done.
Resolving deltas: 100% (17/17), done.
Checking connectivity... done.
2014-08-11 14:57:04,658 DEBUG Checkout ref:772cc75547841c42b9eb71bcffa0904b2329d911 in /tmp/tmpk7D4HJ
2014-08-11 14:57:04,658 DEBUG Executing "git checkout 772cc75547841c42b9eb71bcffa0904b2329d911" in /tmp/tmpk7D4HJ
Note: checking out '772cc75547841c42b9eb71bcffa0904b2329d911'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b new_branch_name

HEAD is now at 772cc75... New tarballs
2014-08-11 14:57:05,262 INFO Build start: ubuntu ('git://github.com/tianon/docker-brew-ubuntu-core', '772cc75547841c42b9eb71bcffa0904b2329d911', 'precise')
2014-08-11 14:57:57,947 INFO Build success: bdf71c2fbbfd (ubuntu:12.04.5)
2014-08-11 14:57:57,953 INFO Build success: bdf71c2fbbfd (ubuntu:12.04)
2014-08-11 14:57:57,963 INFO Build success: bdf71c2fbbfd (ubuntu:precise)
2014-08-11 14:57:57,972 DEBUG Checkout ref:772cc75547841c42b9eb71bcffa0904b2329d911 in /tmp/tmpk7D4HJ
2014-08-11 14:57:57,972 DEBUG Executing "git checkout 772cc75547841c42b9eb71bcffa0904b2329d911" in /tmp/tmpk7D4HJ
HEAD is now at 772cc75... New tarballs
2014-08-11 14:57:58,301 INFO Build start: ubuntu ('git://github.com/tianon/docker-brew-ubuntu-core', '772cc75547841c42b9eb71bcffa0904b2329d911', 'utopic')
2014-08-11 14:59:06,282 INFO Build success: 903020e71007 (ubuntu:14.10)
2014-08-11 14:59:06,288 INFO Build success: 903020e71007 (ubuntu:utopic)
2014-08-11 14:59:06,293 DEBUG Checkout ref:772cc75547841c42b9eb71bcffa0904b2329d911 in /tmp/tmpk7D4HJ
2014-08-11 14:59:06,293 DEBUG Executing "git checkout 772cc75547841c42b9eb71bcffa0904b2329d911" in /tmp/tmpk7D4HJ
HEAD is now at 772cc75... New tarballs
2014-08-11 14:59:06,297 INFO Build start: ubuntu ('git://github.com/tianon/docker-brew-ubuntu-core', '772cc75547841c42b9eb71bcffa0904b2329d911', 'trusty')
2014-08-11 15:00:19,801 INFO Build success: ce52931429c6 (ubuntu:14.04.1)
2014-08-11 15:00:19,808 INFO Build success: ce52931429c6 (ubuntu:14.04)
2014-08-11 15:00:19,813 INFO Build success: ce52931429c6 (ubuntu:trusty)
2014-08-11 15:00:19,818 INFO Build success: ce52931429c6 (ubuntu:latest)
```