Cannot read property 'apiUrl' of undefined

I am trying to create Github instance without authentication

``` javascript
const github = new Github()
```

and having this error

```
Cannot read property 'apiUrl' of undefined
```

I looked through the code and found out that it was fixed (https://github.com/michael/github/blob/master/src/github.js#L46)

Please publish this fix to npm :)
