Don't require people to create roles

If you leave out the username, developers can use their already set-up
postgres role to create the dev database and migrate it. This avoids
forcing them to create a DB role for every project they have in
development. By default postgres uses your username and ident, which
should already be set up by the developer when they installed Postgres
Merge pull request #3 from wjlroe/postgres-user

Don't require people to create roles