Composer improvements
Composer improvements

* PHP requirements
* New constraint notations format
* New stable sonata packages
* Improve conflict section
* Remove minimum-stability rule
* Improve description

Aylarius/SonataPageBundle,sonata-project/SonataPageBundle,Aylarius/SonataPageBundle,Th3Mouk/SonataPageBundle,grimpows/SonataPageBundle,Dunduro/SonataPageBundle,Dunduro/SonataPageBundle,Th3Mouk/SonataPageBundle,sonata-project/SonataPageBundle,Th3Mouk/SonataPageBundle,Dunduro/SonataPageBundle,Aylarius/SonataPageBundle,grimpows/SonataPageBundle,grimpows/SonataPageBundle,Dunduro/SonataPageBundle,grimpows/SonataPageBundle,grimpows/SonataPageBundle,Th3Mouk/SonataPageBundle,Aylarius/SonataPageBundle,Aylarius/SonataPageBundle,Dunduro/SonataPageBundle