Because you missed the libraries that are not including in the Vokuro sample project.
You need to create a composer.json file in your root project likes this:
composer.json
{
"require": {
"swiftmailer/swiftmailer" : "@stable",
"elasticsearch/elasticsearch" : "~1.0",
"aws/aws-sdk-php": "2.4.*@dev"
}
}
and add a file called composer.phar
in the same place with composer.json
then you should run this command to automatically get the libraries that defined in your composer.json
php composer.phar install
For more infomation, please visit this link: https://getcomposer.org/doc/00-intro.md