Rake task to set Commit=true on Avatax SalesInvoices

For anyone that used spree_avatax prior to Commit=true being passed to Avatax automatically.
i.e. before this got pulled in: https://github.com/bonobos/spree_avatax/pull/25

Only the SOAP interface can perform this action in the way we need to in this situation so I also updated to add the SOAP gem + configurations (pulled from 2-2-dev branch which already has that stuff).