Fixes rubocop#862

and fix the build

Switching the cop on by default to match RSpec recommendations from its
docs and the community RSpec Style Guide recommendations.

    `let` _can_ enhance readability when used sparingly (1,2, or
    maybe 3 declarations) in any given example group, but that can
    quickly degrade with overuse. YMMV.

https://github.com/rspec/rspec-core/blob/5d87c28bb828ba14b54a0d8cfddf044894009314/lib/rspec/core/memoized_helpers.rb#L257

https://reddit.com/r/ruby/comments/73nrhk/ama_the_authors_of_effective_testing_with_rspec_3/dnsyanp/

https://rspec.rubystyle.guide/#let-blocks

Example change to dramatically reduce the number of used memoized
helpers without resorting to methods rubocop/rubocop#8447

There is no limit to the number of memoized helpers being used in
practice:

10 lets https://github.com/forem/forem/blob/master/spec/support/api_analytics_shared_examples.rb#L2
20 lets https://github.com/chef/chef/blob/master/spec/unit/provider/systemd_unit_spec.rb#L23
30 lets https://github.com/chef/chef/blob/master/spec/unit/policy_builder/policyfile_spec.rb#L24
40 lets https://github.com/diaspora/diaspora/blob/develop/spec/integration/migration_service_spec.rb#L7
50 lets https://github.com/chef/chef/blob/master/spec/functional/resource/dsc_script_spec.rb#L71

Allowing `subject` by default, i.e. subjects don't count towards the
number of memoized helpers, since there is a dedicated
`RSpec/MultipleSubjects` cop. Combination with it provides infinite
possibilities to tweak the maximum allowed number of memoized helpers to
meet the project style.

Fetching memoized helpers for each of the ancestors is not optimal, is
not optimal and has non-linear complexity.