Updated composer.json

I've corrected the branch alias, and have tweaked the dependencies. I think it would be a good idea to ask for at least symfony 2.1 rather than 2.0. I'd personally like to ask for 2.3, but that would probably be too restrictive and unnecessary. I think we can be sure that nobody is still using symfony 2.0, so ~2.1 would be good. I've also fixed the php parser dependency to prevent this package being broken when 1.0 of that package is released.