fix cwd conflict between masspay and payday

The payday module assumes a certain cwd on import: it tries to read a file at `./fake_payday.sql`. The masspay script was changing the current working directory before (implicitly) importing `gratipay.billing.payday`, causing the import to fail on trying to read the file. This commit works around the issue by moving the import in masspay before the chdir in masspay.