Define MIT license at composer.json

[phansys]

Currently, the package definition does not include the license type.
Since this is a open source package, the license declaration is important to expose how this package can be consumed.

I used the MIT license because this is already used at knplabs/gaufrette.

By instance, I'm using a license checker to confirm that all my dependencies (direct or transitive) comply with my licensing constraints, but the absence of the license declaration in this package blocks its installation.

Please. let me know if you require further context or a different license type, like Unlicense.

BTW, as the same situation seem the same in majority of packages under the gaufrette/ vendor, I'd be glad to provide PRs for the remaining packages if this proposal is accepted.