Implement Nagios-compatible healtcheck

The healthcheck this replaces didn't return a suitable format for Nagios
- the correct format is:

```
{
    "status": "ok",
    "checks": {
        "sufficient_devops": {
            "status": "ok"
        },
        "whizzbangs_recently_frobnicated": {
            "status": "ok"
        }
    }
}
```

which means we need to determine what is `ok`, `warning` etc. within the
code.
Merge pull request #59 from alphagov/fix-healthcheck-format

Implement Nagios-compatible healtcheck