Use subject instead of let(:subject)

Correct a simple oversight. Using `subject` is more idiomatic.

Change-Id: I33e419946c4598d0e28d4bdb30f4ef0a1d57e8b1
Update Rubocop config to ignore more lints

This will reduce the noise.

Change-Id: If7c199ba8f022d876742029cc48ccd2b77c88e56