Fix broken billing statement command tests

The last commit of #1103 broke the tests. I must've forgetten to re-test, sorry guys.
[The test](https://github.com/praekelt/vumi-go/blob/feature/issue-1105-fix-broken-billing-statement-command-tests/go/billing/tests/test_go_gen_statements.py#L57-L68) and running the command locally suggest otherwise. I am surprised by it though.