Report customers in deterministic order

The result could be random before which led to a flaky spec. I could
have adjusted the spec but I thought that it may be better UX as well to
be deterministic, especially when exporting into spreadsheets.

lib/reporting/reports/customers/base.rb

@@ -8,6 +8,7 @@ module Reporting
           filter Spree::Order.managed_by(@user)
             .distributed_by_user(@user)
             .complete.not_state(:canceled)
+            .order(:id)
         end
 
         def filter(orders)