Add OS X support

[thomvaill]

Make the project work on OS X.

Since I don't have a Mac, help is welcome :-)

For this issue, we have to consider to potentially dockerize the CLI + Ansible + Terraform.
However I don't think dockerizing Vagrant is possible.
We have to think about the best option to do this.

[toby-griffiths]

I'd like to use this project for our work environment, and I'm a Mac user, so perhaps I can help. I've initially submitted PR #14 to use Homebrew to install dev dependencies, so let me know if there is still an appetite for this, or perhaps, as you suggest, we Dockorise it.

Regarding the Dockerisation… could the test environment not use Docker-in-Docker as a replacement for Vagrant/Virtualbox?