[2.3] Tweaked requirement settings

changed again some requirement settings in composer.json in order to use mostly stable packages; updated README file.

The only required package that does not exist in a stable version yet is jms/serializer-bundle 0.12 (stable is 0.11). I didn't check if it would work with stable 0.11 because the 0.12 requirement is already in your pre-2.3 master version. Is there really a reason we need 0.12 or could we try to go with 0.11?