Use hatchet.json for config

The new `hatchet.json` stores info of where your external rails/ruby projects that you're testing against are located. This goes along with the new `$ hatchet` CLI.

Declare dependencies in your `hatchet.json` like this:

    {
      "rails3": ["git@github.com:codetriage/codetriage.git"],
      "rails2": ["git@github.com:heroku/rails2blog.git"]
    }

Then run `$ hatchet install` which will clone a directory structure like this:

    repos/
      rails3/
        codetriage/
          #...
      rails2/
        rails2blog/
          # …

Since `hatchet.json` manages these dependencies, you can now just use the name of the repo such as `codetriage` instead of the full path to the repo such as `repos/rails3/codetriage'

After installing repos you should add the directory where your repos live in your `.gitignore` file so they are not required when a buildpack is required in production on Heroku.
Merge pull request #3 from heroku/schneems/json-build

Use hatchet.json for config