Switch to using Ardb's `RelationSpy`

This switches to using Ardb's `RelationSpy` instead of the
custom one in MR. Ardb's was built from the one in MR and behaves
as it did. This properly switches to letting Ardb provide this
instead of MR doing it itself.

Related to #98
Merge pull request #127 from redding/jcr-ardb-relation-spy

Switch to using Ardb's `RelationSpy`