Fix to 'Un-stamped database!' bug

Summary:
Attempting an ./inbox start (for example) after running ./tools/clear-db results in an 'Un-stamped database error';
clearing the db drops the alembic_version table, hence the check for it in inbox:283 fails.

This commit fixes the problem by always including the alembic_version table in the tables kept i.e. not dropped

Test Plan: All scripts work as before.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D82