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.
Definitely! thanks for catching that up, I updated one of the two conditions and forgot about the other ... <_<' I change that right now !