Update composer.json

I've updated the `lluminate/support` to allow use in future version of laravel too. I've updated `phpspec/phpspec` to allow future non-braking versions and removed the dev flag since 2.0 is released now. I have loosed the mockery dependency because any more 0.x releases will not be breaking as the next planned version is 1.0. Finally, I have removed `minimum-stability` because it already defaults to stable.