Based on http://stackoverflow.com/a/4158763/472153:

* brew tap homebrew/boneyard
* cd $( brew --prefix )

* brew versions docker
* git checkout 9ccfc7e Library/Formula/docker.rb
* brew unlink  docker
* brew install docker
* brew switch  docker 1.2.0
* docker --version

* brew versions boot2docker
* git checkout 5cfea87 Library/Formula/boot2docker.rb
* brew unlink  boot2docker
* brew install boot2docker
* brew switch  boot2docker 1.2.0
* boot2docker version

* wget https://github.com/boot2docker/boot2docker/releases/download/v1.2.0/boot2docker.iso -O ~/.boot2docker/boot2docker.iso
* boot2docker restart