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.

I've also made a few improvements to your version constraints.

One other thing I noticed is that one of your test files has left over diff stuff in it, so is broken.
https://github.com/UlrickAsura/taxonomy/blob/master/tests/TaxonableCase.php