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.
Bump Rails to 4.1.7.1

This is a security fix update (though we don't believe we were vulnerable).

https://github.com/alphagov/email-alert-api/pull/54 attempted to update
rails to 4.1.8, but that release included other changes which caused our
build to break (due to problems setting up the postgres hstore
extension).

4.1.7.1 was then released, containing just the security fixes.
Merge pull request #58 from alphagov/bump-rails-to-4-1-7-1

Bump Rails to 4.1.7.1
Merge pull request #59 from alphagov/fix-healthcheck-format

Implement Nagios-compatible healtcheck