lib/plugins/aws/rollbackFunction/index.js/AwsRollbackFunction/restoreFunction
class AwsRollbackFunction:  restoreFunction(zipBuffer) {
    const funcName = this.options.function;

    this.serverless.cli.log('Restoring function...');

    const funcObj = this.serverless.service.getFunction(funcName);

    const params = {
      FunctionName: funcObj.name,
      ZipFile: zipBuffer,
    };

    return this.provider.request(
      'Lambda',
      'updateFunctionCode',
      params,
      this.options.stage, this.options.region
    ).then(() => {
      this.serverless.cli.log(`Successfully rolled back function "${this.options.function}"`);
    });
  }