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.
Merge pull request #2876 from gratipay/fix-masspay-for-cwd

fix cwd conflict between masspay and payday