Provide brew installer

[balupton]

Would be great if I could just do:

brew tap phinze/cask
brew install brew-cask
brew cask install vagrant virtualbox
brew install docker

[shurane]

+1. This is straightforward to implement.

[tt]

Ideally, this could also take a dependency on the Docker formula making it both unnecessary to install as part of docker-osx and therefore unnecessary to update the script whenever a new, compatible version is released.