Change to droplet_kit for API

[petems]

Right now we haven't kept parity with DigitalOcean's new API features in the API gem we're using Barge (See #249, #248)

At the time we moved, DigitalOcean didn't have a Gem for the API.

Now there is! https://github.com/digitalocean/droplet_kit/

The API for accessing is almost 1:1, plus it's maintained by DO themselves. It would make sense to move over to this so we don't have to implement all the new endpoints in barge.

[petems]

Currently blocked on exposing faraday: digitalocean/droplet_kit#102

[petems]

Starting move on some of the larger code blocks with the merge of #284 and the bump to 4.0.0