Makefile.PL now pulls in DBIC deploy deps

https://metacpan.org/pod/distribution/DBIx-Class/lib/DBIx/Class/Optional/Dependencies.pod
selling_price in ResultSet::Product will always return defined price

In order to be able to sort products based on selling_price it is not
acceptable for selling_price to return undef values. We now set
selling_price to price if selling_price would be undef.