Fix serializer relationship
Fix serializer relationship

Since `root` died I don't know what we would use to fix this
call to relationship. There might be a simpler way to fix this
but since it's an old syntax only supported for compatibility with
0.8.0 the docs are lame and I don't feel like digging around the
source.

solidusio/solidus_signifyd,solidusio/solidus_signifyd