show a warning when github is down

![screen shot 2014-10-08 at 5 31 13 pm](https://cloud.githubusercontent.com/assets/13248/4569529/bb5ec112-4f4b-11e4-9c25-5024ea4f8c4e.png)

Not really sure if this belongs in a helper or it's own class at this point, but whatever.

#29 

/cc @zendesk/samson @grosser 
no need for local assignment:
```Ruby
Rails.cache.fetch(github_status_cache_key, expires_in: 5.minutes) do
  response = Faraday.get("https://#{status_url}/api/status.json")
  (response.status == 200 && JSON.parse(response.body)['status'] == 'good') || nil  # don't cache bad responses
end
```