def write_manifest
      generator = ManifestGenerator.new(spec)

      File.open(@output_dir.join('manifest.json'), 'w') do |f|
        f.write JSON.pretty_generate(generator.spec_as_json)
      end
    end
def write_manifest
      generator = ManifestGenerator.new(spec)

      File.open(@output_dir.join('manifest.json'), 'w') do |f|
        f.write JSON.pretty_generate(generator.spec_as_json)
      end
    end
Generate the manifest from the spec and write it to disk