Update composer.json

Hi there. I'm reaching out to everybody who I can see is using "zizaco/factory-muff".
This package has been moved over to "league/factory-muffin".

See: https://github.com/thephpleague/factory-muffin.

Also, I noticed you were using dev-master for version constraints. That's a terrible thing to do. I've corrected that for you too.