Fix license definition in composer.json

Summary:
The license has to be defined in a predefined format.
For more infos see https://getcomposer.org/doc/04-schema.md#license

Reviewers: dkd-doan, #forgetit-dev

Reviewed By: dkd-doan, #forgetit-dev

Differential Revision: https://phabricator.dkd.de/D380