Add RSpec 4 compatibility (#17652)

* Remove outdated option

https://github.com/rspec/rspec-core/commit/04078317ba6577699d06cf4dccf014254dcde7a6

* Use the non-globally exposed RSpec syntax

https://github.com/rspec/rspec-core/pull/2803

* Use the non-globally exposed RSpec syntax, cont

https://github.com/rspec/rspec-core/pull/2803

* Comply to strict predicate matchers

See:
 - https://github.com/rspec/rspec-expectations/pull/1195
 - https://github.com/rspec/rspec-expectations/pull/1196
 - https://github.com/rspec/rspec-expectations/pull/1277

spec/lib/timeline_lookup_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe TimelineLookup do
+RSpec.describe TimelineLookup do
   it "returns an empty array for empty input" do
     expect(TimelineLookup.build([])).to eq([])
   end