Deprecate plan["collaborators"] attribute

Personal plans and organization plans used to allow a finite number of collaborators. The API used to return the allowed number of collaborators for each plan in the `collaborators` attribute:

```
{
  "login": "jasonrudolph",
  ...
  "plan": {
    "name": "medium",
    "space": 2516582,
    "collaborators": 10,
    "private_repos": 20
  }
}
```

Now, these [plans allow unlimited collaborators](https://github.com/pricing). As a result, this attribute is no longer meaningful. We will remove it in the next major version of the API. In the meantime, for backwards compatibility purposes, the v3 API and the beta API continue to provide an integer value (`0`) for the attribute.

```
{
  "login": "jasonrudolph",
  ...
  "plan": {
    "name": "medium",
    "space": 2516582,
    "collaborators": 0,
    "private_repos": 20
  }
}
```

This PR adds this attribute to the [list of v3 deprecations](https://developer.github.com/v3/versions/#v3-deprecations), and it updates the example response to match current functionality.