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