Handle load paths the same way in health check.

In my previous commit (3bcbb91b), I said there were three ways of running code
from Rummager. As it turns out, there are four main ways.
Resolve a circular dependency in indexing worker.
Make all the load path code use the same syntax.

Brackets are clearer, and `$LOAD_PATH` is clearer than `$:`.

Hat tip to @jamiecobbett.
Merge pull request #148 from alphagov/fix-health-check

Fix health check script
Revert "Add fallback for registry entries without slugs."

Since the slugs are now migrated, we don't need this fallback any more.

This reverts commit ca08b45af9bfeac7e89d13e14b70c056a109afd3.
Merge pull request #149 from alphagov/remove-slug-fallback

Revert "Add fallback for registry entries without slugs."