keep yaml as default cassette serializer to fix unit tests
keep yaml as default cassette serializer to fix unit tests

kevin1024/vcrpy,poussik/vcrpy,IvanMalison/vcrpy,graingert/vcrpy,bcen/vcrpy,mgeisler/vcrpy,ByteInternet/vcrpy,yarikoptic/vcrpy,ByteInternet/vcrpy,agriffis/vcrpy,graingert/vcrpy,poussik/vcrpy,kevin1024/vcrpy,gwillem/vcrpy,aclevy/vcrpy