Shippable support for continuous builds

None of the hosted continuous integration solutions I've been able to find provide the ability to run your actual docker containers. 

Shippable runs _on_ docker and it's very fast compared to Travis or Circle. They've put their own layer of abstraction over docker - the `shippable.yml` file - which defines how the build machine is set up.

A full CI pipeline using shippable, docker and ansible is described in [this article](http://www.hiddentao.com/archives/2014/06/03/shippable-ansible-docker-loggly-for-awesome-deployments/). For now, considering we don't have anywhere to deploy to, I've chosen only to set up the build node.

That uses leiningen to run the tests in the project gives a status icon for the build. I think it will also build pull requests automatically so you can check the build status before a merge. They are planning a closer integration with GitHub for inline status icons in pull requests, I seem to remember seeing it on their roadmap.