Dependent: destroy linking Spree::Variant and Spree::WishedProduct

This is the second half of addressing https://github.com/spree/spree_wishlist/issues/65#issuecomment-39400377 -- the first being a migration to remove phantom `WishedProducts`

Fixes #66
Migration to cull wished_products with nil variant

First half of change that addresses https://github.com/spree/spree_wishlist/issues/65#issuecomment-39400377

Fixes #66