Add missing license information in composer.json

The license information was missing from the composer.json file. This information is quite important as it is displayed on Packagist and used by automated tools (like http://insight.sensiolabs.com/ for instance) to check compatibility of your project dependencies.
Merge pull request #2278 from fabpot/patch-1

Add missing license information in composer.json
Merge branch 'master' of https://github.com/laravel/laravel
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