Close the old index after migration.

There's no way to reach it from within the app, and rollbacks can only be done
manually, so there's no reason to keep the index open and taking up resources
on the elasticsearch server.
Use an inclusive range for clarity.

When the value is `400...500`, you have to stop and think whether that
includes the 500 error code; when it's `400..499`, while the effect is
the same, it's a little more obvious what the intention is.

Hat tip to @jamiecobbett.
Merge pull request #155 from alphagov/close-after-migration

Close the old index after migration.