Fix mismatch between composer.lock and composer.json

The composer.lock file that was committed to the repository did not
match composer.json. It was built from a composer.json with an extra
section to override the PHP version check:

    "config": {
        "platform": {
            "php": "7.1.33"
        }
    }

Rebuild composer.lock without that config section, to prevent warnings
from Composer due to the mismatch.