Various instrumentation improvements and updates

Summary:

* changed the webserver to `puma`
* upgraded gems to their latest version
* set the backend for `librato-metrics` to use `excon`
* standardized on `excon` for external calls, removed `rest-client`
* instrument `excon`
* added `request_id` context to logs and return json payloads
* cleaned up some logging stuff in `/check` path