Disambiguate references to Set class

When run in the gem context, the references to the `Set` class were
thought to be under the `Overcommit` module. Fix this by explicitly
requiring the `set` library.

Change-Id: I5cd1c683b9e7fd64f3e7e7aa610b5e28319f6ee1
Reviewed-on: http://gerrit.causes.com/35638
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>