Configuration for php-composer-validate

We only need to run 'composer validate' when a change tweak the
/composer.json file.

Job created by https://gerrit.wikimedia.org/r/136726

Change-Id: I6bfe82de142c16ce95636ee76254e74bce1e5d5a
Validate composer file in mediawiki/core/vendor

Change-Id: Ia46840e930fd03ce7747c7d2c1850b48d43d514b