Add ruby image

```console
root@369e523f335e:/stackbrew/stackbrew# ./brew-cli --debug -b ruby --targets ruby git://github.com/infosiftr/stackbrew
2014-06-04 22:15:03,625 DEBUG Cloning repo_url=git://github.com/infosiftr/stackbrew, ref=ruby
2014-06-04 22:15:03,627 DEBUG folder = /tmp/tmp4OkBQM
2014-06-04 22:15:03,627 DEBUG Cloning git://github.com/infosiftr/stackbrew into /tmp/tmp4OkBQM
2014-06-04 22:15:03,628 DEBUG Executing "git clone git://github.com/infosiftr/stackbrew ." in /tmp/tmp4OkBQM
Cloning into '.'...
remote: Counting objects: 739, done.
remote: Compressing objects: 100% (385/385), done.
remote: Total 739 (delta 404), reused 643 (delta 343)
Receiving objects: 100% (739/739), 111.99 KiB | 0 bytes/s, done.
Resolving deltas: 100% (404/404), done.
Checking connectivity... done.
2014-06-04 22:15:04,715 DEBUG Checkout ref:ruby in /tmp/tmp4OkBQM
2014-06-04 22:15:04,715 DEBUG Executing "git checkout ruby" in /tmp/tmp4OkBQM
Branch ruby set up to track remote branch ruby from origin.
Switched to a new branch 'ruby'
2014-06-04 22:15:04,720 DEBUG latest: git://github.com/docker-library/ruby@cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4

2014-06-04 22:15:04,721 DEBUG 2: git://github.com/docker-library/ruby@cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4

2014-06-04 22:15:04,721 DEBUG 2.1: git://github.com/docker-library/ruby@cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4

2014-06-04 22:15:04,721 DEBUG 2.1.2: git://github.com/docker-library/ruby@cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4

2014-06-04 22:15:04,721 DEBUG ruby: latest,2,2.1,2.1.2
2014-06-04 22:15:04,721 DEBUG Cloning repo_url=git://github.com/docker-library/ruby, ref=cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4
2014-06-04 22:15:04,721 DEBUG folder = /tmp/tmpqahCZ0
2014-06-04 22:15:04,721 DEBUG Cloning git://github.com/docker-library/ruby into /tmp/tmpqahCZ0
2014-06-04 22:15:04,721 DEBUG Executing "git clone git://github.com/docker-library/ruby ." in /tmp/tmpqahCZ0
Cloning into '.'...
remote: Counting objects: 209998, done.
remote: Compressing objects: 100% (46642/46642), done.
remote: Total 209998 (delta 161675), reused 209998 (delta 161675)
Receiving objects: 100% (209998/209998), 86.48 MiB | 1.18 MiB/s, done.
Resolving deltas: 100% (161675/161675), done.
Checking connectivity... done.
2014-06-04 22:17:18,505 DEBUG Checkout ref:cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4 in /tmp/tmpqahCZ0
2014-06-04 22:17:18,505 DEBUG Executing "git checkout cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4" in /tmp/tmpqahCZ0
Note: checking out 'cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4'.

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 cbf7ec2... Add initial Dockerfile
2014-06-04 22:17:18,839 INFO Build start: ruby ('git://github.com/docker-library/ruby', 'cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4', '.')
2014-06-04 22:22:19,121 ERROR Build failed for ruby (('git://github.com/docker-library/ruby', 'cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4', '.'))
Traceback (most recent call last):
  File "/stackbrew/stackbrew/brew/brew.py", line 172, in build_repo
    self.build_version(repo, version, callback)
  File "/stackbrew/stackbrew/brew/brew.py", line 195, in build_version
    repo, version, dockerfile_location, callback
  File "/stackbrew/stackbrew/brew/brew.py", line 284, in do_build
    raise exc
StackbrewError: Build failed for ruby (('git://github.com/docker-library/ruby', 'cbf7ec20fbc40ddac9259c591c9d5e48a86d29e4', '.'))
root@369e523f335e:/stackbrew/stackbrew# ./brew-cli --debug -b ruby --targets ruby git://github.com/infosiftr/stackbrew
2014-06-04 22:22:29,456 DEBUG Cloning repo_url=git://github.com/infosiftr/stackbrew, ref=ruby
2014-06-04 22:22:29,459 DEBUG folder = /tmp/tmpUp_SPG
2014-06-04 22:22:29,459 DEBUG Cloning git://github.com/infosiftr/stackbrew into /tmp/tmpUp_SPG
2014-06-04 22:22:29,459 DEBUG Executing "git clone git://github.com/infosiftr/stackbrew ." in /tmp/tmpUp_SPG
Cloning into '.'...
remote: Counting objects: 743, done.
remote: Compressing objects: 100% (389/389), done.
remote: Total 743 (delta 407), reused 643 (delta 343)
Receiving objects: 100% (743/743), 112.31 KiB | 0 bytes/s, done.
Resolving deltas: 100% (407/407), done.
Checking connectivity... done.
2014-06-04 22:22:30,351 DEBUG Checkout ref:ruby in /tmp/tmpUp_SPG
2014-06-04 22:22:30,351 DEBUG Executing "git checkout ruby" in /tmp/tmpUp_SPG
Branch ruby set up to track remote branch ruby from origin.
Switched to a new branch 'ruby'
2014-06-04 22:22:30,358 DEBUG latest: git://github.com/docker-library/ruby@15a75728df122ca5e0210c296095285facda11d4

2014-06-04 22:22:30,358 DEBUG 2: git://github.com/docker-library/ruby@15a75728df122ca5e0210c296095285facda11d4

2014-06-04 22:22:30,358 DEBUG 2.1: git://github.com/docker-library/ruby@15a75728df122ca5e0210c296095285facda11d4

2014-06-04 22:22:30,358 DEBUG 2.1.2: git://github.com/docker-library/ruby@15a75728df122ca5e0210c296095285facda11d4

2014-06-04 22:22:30,358 DEBUG ruby: latest,2,2.1,2.1.2
2014-06-04 22:22:30,358 DEBUG Cloning repo_url=git://github.com/docker-library/ruby, ref=15a75728df122ca5e0210c296095285facda11d4
2014-06-04 22:22:30,358 DEBUG folder = /tmp/tmpUd_YiD
2014-06-04 22:22:30,358 DEBUG Cloning git://github.com/docker-library/ruby into /tmp/tmpUd_YiD
2014-06-04 22:22:30,359 DEBUG Executing "git clone git://github.com/docker-library/ruby ." in /tmp/tmpUd_YiD
Cloning into '.'...
remote: Counting objects: 209998, done.
remote: Compressing objects: 100% (46643/46643), done.
remote: Total 209998 (delta 161683), reused 209986 (delta 161674)
Receiving objects: 100% (209998/209998), 86.45 MiB | 1.32 MiB/s, done.
Resolving deltas: 100% (161683/161683), done.
Checking connectivity... done.
2014-06-04 22:24:42,462 DEBUG Checkout ref:15a75728df122ca5e0210c296095285facda11d4 in /tmp/tmpUd_YiD
2014-06-04 22:24:42,462 DEBUG Executing "git checkout 15a75728df122ca5e0210c296095285facda11d4" in /tmp/tmpUd_YiD
Note: checking out '15a75728df122ca5e0210c296095285facda11d4'.

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 15a7572... Add initial Dockerfile
2014-06-04 22:24:42,740 INFO Build start: ruby ('git://github.com/docker-library/ruby', '15a75728df122ca5e0210c296095285facda11d4', '.')
2014-06-04 22:36:09,261 INFO Build success: 7b540bba0850 (ruby:latest)
2014-06-04 22:36:09,266 INFO Build success: 7b540bba0850 (ruby:2)
2014-06-04 22:36:09,288 INFO Build success: 7b540bba0850 (ruby:2.1)
2014-06-04 22:36:09,291 INFO Build success: 7b540bba0850 (ruby:2.1.2)
```
(cc @yosifkit)