lib/plugins/aws/rollback/index.js/AwsRollback/constructor
class AwsRollback:  constructor(serverless, options) {
    this.serverless = serverless;
    this.options = options;
    this.provider = this.serverless.getProvider('aws');

    Object.assign(
      this,
      validate,
      setBucketName,
      updateStack,
      monitorStack
    );

    this.hooks = {
      'before:rollback:initialize': () => BbPromise.bind(this)
        .then(this.validate),
      'rollback:rollback': () => {
        if (!this.options.timestamp) {
          const command = this.serverless.pluginManager.spawn('deploy:list');
          this.serverless.cli.log([
            'Use a timestamp from the deploy list below to rollback to a specific version.',
            'Run `sls rollback -t YourTimeStampHere`',
          ].join('\n'));
          return command;
        }

        return BbPromise.bind(this)
          .then(this.setBucketName)
          .then(this.setStackToUpdate)
          .then(this.updateStack);
      },
    };
  }