Have CreditCardAccount.need_reconciling return an ActiveRecord::Relation instead of an Array

This should resolve the recent spec failures.

Initially I wrote a smaller patch that changed the code to expect an `Array` and left `need_reconciling` alone*, but it feels better to me if it returns a relation.

* 59719fd2d6816e7a2c97e9df730a9be747c4b19d...5b3bd720c9cea4b20801bb1146c415531b4e3841 (I deleted the branch so this diff may only work until the next git garbage collection)