Require docker-api in Gemfile

In lib/fog/fogdocker/compute.rb, require 'docker' is used.
The gem is used throughout this part of the code but is not required in the Gemfile,
I've added it, it's https://github.com/swipely/docker-api
Merge pull request #3188 from eLobato/docker-api-gem

Require docker-api in Gemfile