Update database Terraform module once the max_allocated_storage PR is merged

[KlaasH]

PR #1312 switches the Terraform module we're using for the database configuration to use a fork, since we're in a hurry. I made a PR to merge the forked version into the upstream module (azavea/terraform-aws-postgresql-rds#48), so once that (or something like it) has been merged and a new release tagged, we should switch the module source back to that repo with the new tag.