chore(builder): migrate to cedar-14 stack

With quantal coming out-of-support, heroku implemented the cedar-14
stack. This stack is what's used on heroku in production today AFAIK.
Migrating over to this stack makes us more compatible with Heroku.

TODO:
 - [ ] snapshot progrium/cedarish to master for #1047 and for https://github.com/progrium/cedarish/pull/9

Test run to ensure we don't hit https://github.com/deis/deis/issues/1742 again:

```
><> cd example-ruby-sinatra/
><> deis create
Creating application... done, created timely-zoologer
Warning: Permanently added '[deis.local.deisapp.com]:2222,[172.17.8.100]:2222' (RSA) to the list of known hosts.
Git remote deis added
><> git push deis master
Warning: Permanently added '[deis.local.deisapp.com]:2222,[172.17.8.100]:2222' (RSA) to the list of known hosts.
Counting objects: 102, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (53/53), done.
Writing objects: 100% (102/102), 22.19 KiB | 0 bytes/s, done.
Total 102 (delta 43), reused 102 (delta 43)
-----> Ruby app detected
-----> Compiling Ruby/Rack
-----> Using Ruby version: ruby-1.9.3
-----> Installing dependencies using 1.6.3
       Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
       Don't run Bundler as root. Bundler can ask for sudo if it is needed, and
       installing your bundle as root will break this application for all non-root
       users on this machine.
       Fetching gem metadata from http://rubygems.org/..........
       Using bundler 1.6.3
       Installing rack 1.5.2
       Installing rack-protection 1.5.0
       Installing tilt 1.3.6
       Installing sinatra 1.4.2
       Your bundle is complete!
       Gems in the groups development and test were not installed.
       It was installed into ./vendor/bundle
       Bundle completed (4.59s)
       Cleaning up the bundler cache.

-----> Discovering process types
       Procfile declares types -> web
       Default process types for Ruby -> rake, console, web
-----> Compiled slug size is 55M
remote: -----> Building Docker image
remote: Sending build context to Docker daemon 57.22 MB
remote: Sending build context to Docker daemon
remote: Step 0 : FROM deis/slugrunner
remote:  ---> ae8c68dd19ad
remote: Step 1 : RUN mkdir -p /app
remote:  ---> Running in 569eedd79dd8
remote:  ---> b6ab32f5a8d6
remote: Removing intermediate container 569eedd79dd8
remote: Step 2 : WORKDIR /app
remote:  ---> Running in 40d8e02558d6
remote:  ---> f58b84b63338
remote: Removing intermediate container 40d8e02558d6
remote: Step 3 : ENTRYPOINT ["/runner/init"]
remote:  ---> Running in 40f39570128f
remote:  ---> 763e998a26b2
remote: Removing intermediate container 40f39570128f
remote: Step 4 : ADD slug.tgz /app
remote:  ---> 7ac39867c24d
remote: Removing intermediate container 9245471e01ab
remote: Successfully built 7ac39867c24d
remote: -----> Pushing image to private registry
remote:
remote:        Launching... done, v2
remote:
remote: -----> timely-zoologer deployed to Deis
remote:        http://timely-zoologer.local.deisapp.com
remote:
remote:        To learn more, use `deis help` or visit http://deis.io
remote:
To ssh://git@deis.local.deisapp.com:2222/timely-zoologer.git
 * [new branch]      master -> master
><> deis run env
HOSTNAME=c569433d22bc
DEIS_APP=timely-zoologer
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/app
SHLVL=1
HOME=/root
PORT=5000
DEIS_RELEASE=v2
_=/usr/bin/env
><> deis run echo hi
hi
><> deis run uname -a
-a requires argument
Usage: deis apps:run [options] [--] <command>...
><> deis run --uname -a
-a requires argument
Usage: deis apps:run [options] [--] <command>...
><> deis run -- uname -a
Linux ffa8ecbd27bd 3.16.2+ #2 SMP Fri Sep 26 17:16:13 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
><> deis run -- bash --version
GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
```

closes #1914