Two-factor auth

For accounts that require two-factor auth, it is not enough to provide `--username` and `--password`.

``` json
{
  "message": "Must specify two-factor authentication OTP code.",
  "documentation_url":"https://developer.github.com/v3/auth#working-with-two-factor-authentication"
}
```

Would this require changes in the upstream https://github.com/pwmckenna/node-travis-ci project?
