Add support for Docker

[ellneal]

This PR adds a Dockerfile and docker-compose.yml to the repository, as well as providing instructions in the documentation of how to run Heaven using docker. (These should be updated to point to a public Docker image built from this repository; currently it is built from my fork).

The tests can now be run inside a container by running the following command from the project directory:

$ docker-compose up test

Other minor changes (I can move these into a separate PR if necessary):

• Updated the Resque gem. The previously embedded version was resulting in a lot of hanging workers when used with Docker, fixed in Workers register a heartbeat resque/resque#763.
• Defaulted the notifier to a noop. Without a chat token specified, each deployment would result in a failed notifier job, which it no longer does.
• Added ability to restrict access to members of a GitHub organisation. The organisation name can be provided using the GITHUB_ORG environment variable. This also fixed a bug where the routing would fail if a GITHUB_TEAM_ID environment variable was not specified.

[atmos]

Thanks so much for this. I'm traveling right now and my docker setup is pretty screwed up. If my docker for mac beta invite comes through while I'm traveling I can test it. Otherwise I'll test it when I'm home in a couple weeks.

[ellneal]

No rush, just wanted feed my amendments back upstream 👍

I've got this running on a Debian box with hubot-deploy and it works nicely.