Capistrano Provider

This supports basic capistrano stuff, I helped a friend setup and deploy a heaven today using it.

This relies on a few things.

* The payload config needs a provider set to `capistrano`
* Only works with capistrano 2.9.x atm
* Supports multiple capistrano deployment tasks via the task attribute on payload. `deploy:migrate` etc.
* Requires an ssh key in the environment `DEPLOYMENT_PRIVATE_KEY` that writes to disk and uses for authentication.
* Requires your `repository` attribute in your capfile to clone over https instead of git+ssh. We use the existing github token to clone over oauth.

An example deployments API payload looks like this.

```javascript
 "payload": {
    "name": "website",
    "task": "deploy",
    "hosts": "",
    "notify": {
      "room": "testing",
      "user": "atmos",
      "adapter": "slack"
    },
    "config": {
      "provider": "capistrano",
      "repository": "myorg/website",
      "environments": [
        "production"
      ]
    }
  }
```

There's a flakey test in here that I'll fix in a subsequent PR. I think it was introduced with the octokit 3.1.0 upgrade.