Improve return type YARD for DiffClusterFinder#clusters

This is more descriptive, which should improve the documentation for
this method.

Change-Id: I6e730e0ee3188e87cb6bf878f6d3f91317539add
Add rubocop config

This configuration file should help us configure rubocop so it behaves
more like we'd like it to. I started with a few options that jumped out
at me, but I expect that this will evolve over time.

Change-Id: I4fdc6081ed8a43038e167d5b2a7b552aa99af5fe