Symfony composer install - Uncaught Symfony\Component\Debug\Exception\ClassNotFoundException: Attempted to load class "SensioGeneratorBundle"

In Prod environnment, the SensioGeneratorBundle is not registred. This bundle should be defined like a dev bundle as follow :

if (in_array($this->getEnvironment(), array('dev', 'test'))) {
        $bundles[] = new Symfony\Bundle\DebugBundle\DebugBundle();
        $bundles[] = new Symfony\Bundle\WebProfilerBundle\WebProfilerBundle();
        $bundles[] = new Sensio\Bundle\DistributionBundle\SensioDistributionBundle();
        $bundles[] = new Sensio\Bundle\GeneratorBundle\SensioGeneratorBundle();
    }

And inside your composer file :

"require-dev": {
    "sensio/generator-bundle": "~2.3"
}

If you want to use generation commands from this bundle in Prod env, you should move the declaration from dev to Prod (not recommended!)