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* Remove outdated option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/oneboxer_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Oneboxer do
+RSpec.describe Oneboxer do
   def response(file)
     file = File.join("spec", "fixtures", "onebox", "#{file}.response")
     File.exist?(file) ? File.read(file) : ""