Composer.json ignored

[gazpachu]

I'm trying to launch with PHP 5.3.23 but no matter what I set in the composer file, it gets ignored :-/

{
"require": {
"php": ">=5.3.23"
},
"engines": {
"php": "5.3.23",
"nginx": "1.3.14"
}
}

remote: Building source:
remote:
remote: -----> Fetching set buildpack https://github.com/CHH/heroku-buildpack-php... done
remote: -----> PHP (classic) app detected
remote: -----> Bundling NGINX 1.4.4
remote: -----> Bundling PHP 5.5.24
remote: -----> Bundling extensions
remote: apcu
remote: phpredis
remote: mongo
remote: ! Script error in /tmp/buildpack20160101-161-125awq9/bin/compile on or near line 380
remote:
remote: ! Push rejected, failed to compile PHP (classic) app
remote:
remote: Verifying deploy...
remote:
remote: ! Push rejected to ....

[fagianijunior]

Do you has a composer.lock?
The README / Detection section says: 'This buildpack detects apps when the app has a composer.lock in the app's root.'