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.
Fix my changes.
Merge pull request #11 from gnutix/master

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