composer.json

{
    "scripts": {
        "post-install-cmd": [
            // put this before installAssets
            "Fmdb\\CoreBundle\\Composer\\ScriptHandler::installBowerAssets",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets"
        ],
        "post-update-cmd": [
            // put this before installAssets
            "Fmdb\\CoreBundle\\Composer\\ScriptHandler::installBowerAssets",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets"
        ]
    },
}

ScriptHandler.php

namespace Fmdb\CoreBundle\Composer;

use Sensio\Bundle\DistributionBundle\Composer\ScriptHandler as BaseScriptHandler;
use Symfony\Component\ClassLoader\ClassCollectionLoader;
use Symfony\Component\Process\Process;

class ScriptHandler extends BaseScriptHandler
{
    public static function installBowerAssets($event)
    {
        $process = new Process('bower install', null, null, null, 300);
        $process->run(function ($type, $buffer) { echo $buffer; });
        if (!$process->isSuccessful()) {
            throw new \RuntimeException(sprintf('An error occurred when executing the "%s" command.', escapeshellarg($cmd)));
        }
    }
}