"licenses" field is misused in package.json

Now "licenses" field of package.json [contains single "MIT" string](https://github.com/react-component/util/blob/master/package.json#L21)

According to [spec](https://docs.npmjs.com/files/package.json#license), the "license" (not "licenses") field should be used to specify the single "MIT" license.

So, field should be renamed "licenses" => "license" I believe.

Thanks!