build output.log

=====> Downloading Buildpack: https://github.com/heroku/heroku-buildpack-php.git
=====> Detected Framework: PHP
-----> No runtime requirements in composer.json, defaulting to PHP 5.6.3.
-----> Installing system packages...
       - PHP 5.6.3
       - Apache 2.4.10
       - Nginx 1.6.0
-----> Installing PHP extensions...
       - zend-opcache (automatic; bundled, using 'ext-zend-opcache.ini')
-----> Installing dependencies...
       Composer version 1.0-dev (2131c8b48082a176565856d69c6273047c96a6dc) 2014-12-09 22:40:08

 !     WARNING: You have put Composer's vendor directory under version control.
       That directory should not be in your Git repository; only composer.json
       and composer.lock should be added, with Composer handling installation.
       Please 'git rm --cached vendor/' to remove the folder from your index,
       then add '/vendor/' to your '.gitignore' list to remove this notice.
       For more information, refer to the Composer FAQ: http://bit.ly/1rlCSZU

       Loading composer repositories with package information
       Installing dependencies
       Nothing to install or update
       Generating optimized autoload files
       Compiling CSS with compass
<internal:lib/rubygems/custom_require>:29:in `require': no such file to load -- bundler/setup (LoadError)
	from <internal:lib/rubygems/custom_require>:29:in `require'
	from vendor/bundle/bin/compass:14:in `<main>'
-----> Preparing runtime environment...
Using release configuration from last framework (PHP).
-----> Discovering process types
       Procfile declares types -> web

installer.php

<?php
namespace App;

use Composer\Script\Event;

class Installer
{
	public static function postInstall(Event $event)
	{
		$io = $event->getIO();
		$io->write('Compiling CSS with compass');

		// Compile CSS with compass
		exec('vendor/bundle/bin/compass compile -c config.rb public/');
	}
}