Platform: Implement monitoring endpoints - GET subscriptions [#77728418]

TODO: describe your changes

===

Pivotal tracker story [#77728418](https://www.pivotaltracker.com/story/show/77728418) in project *Platform*:

> https://github.com/HouseTrip/routemaster/issues/12
> 
> **TEST PLAN**
> 
> set off a sync and check out 
> https://routemaster-staging.herokuapp.com/subscriptions
@marcusleemitchell with this:

```
payload = [Routemaster::Models::Subscription.map do |subscription|
  {
    ...
  }
end]
payload.to_json
```

you'll get:

```
[[{"subscriber"=>"charlie", "callback"=>nil, "topics"=>["widget"], "events"=>{"sent"=>100, "queued"=>50, "oldest"=>1000}}]]
```

(double array).

Your spec is wrong because of `resp = JSON(last_response.body)[0]`.