Add Surf as a CI source
user
```ruby
# http://github.com/surf-build/surf

module Danger
  module CISource
    class Surf < CI
      def self.validates?(env)
        return ['SURF_REPO', 'SURF_NWO'].all? {|x| env[x]}
      end

      def supported_request_sources
        @supported_request_sources ||= [Danger::RequestSources::GitHub]
      end

      def initialize(env)
        self.repo_slug = env['SURF_NWO']
        if env['SURF_PR_NUM'].to_i > 0
          self.pull_request_id = env['SURF_PR_NUM']
        end
        
        self.repo_url = env['SURF_REPO'];
      end
    end
  end
end
```
assistant