Adding dependency management to the gemspec

- Used bundle init to create a Gemfile for the existing gemspec
- Used spec.add_development_dependency to add the gem's dependency on the rest-client gem
- Now using environment variable rather than config.yml file to store Digital Ocean token. This allows for testing on CircleCI without compromising credentials.

References:
http://guides.rubygems.org/specification-reference/#add_runtime_dependency