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
I wanted to enforce the enclosing Array
```
expected: [{"subscriber"=>"charlie", "callback"=>nil, "topics"=>["widget"], "events"=>{"sent"=>100, "queued"=>50, "oldest"=>1000}}]
            got: {"subscriber"=>"charlie", "callback"=>nil, "topics"=>["widget"], "events"=>{"sent"=>100, "queued"=>50, "oldest"=>1000}}
```