v0.0.15

Added support for Digitalocean API V2 (V1 still default).
Added -p option to select API protocol version.
Added --digital-ocean-access-token option.
Updated -h description.
Decline Ruby version earlier than 2.0.0.
Decline JRuby version earlier than 9.0.0.0.

Refactored ::Api into modular interface.
Upgraded Digitalocean gem into DigitaloceanC, for new Faraday.
Added droplet_kit dependency.
Added tests for the new interfaces.