Fix auto-loading with Composer "bin-dir" option

When using Composer "bin-dir" to another folder than vendor/bin, the autoloading of centipede breaks.
The code used in this PR has been taken from SensioLabs' security-checker binaries, to ensure maximum correctness.
It should be `if ((!$loader = includeIfExists(__DIR__.'/../vendor/autoload.php')) && (!$loader = includeIfExists(__DIR__.'/../../../autoload.php'))) {`. Right?