Allow no repo to be specified for /status

This PR allows the user to [omit the repo name](http://rubular.com/r/uuVFSH6etZ).

`githubot` will [automatically assign the repo](https://github.com/iangreenleaf/githubot/blob/master/githubot.coffee#L14-L18) when it's specified in the ENV. Currently set to `jekyll/jekyll` on our instance.

This means the following are equivalent:

```text
/status jekyll/jekyll 2004
/status 2004
```

iff `HUBOT_GITHUB_REPO` is set to `jekyll/jekyll`.