Add RSpec 4 compatibility (#17652)

* Remove outdated option

04078317ba

* 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/onebox/json_ld_spec.rb

@@ -2,7 +2,7 @@
 
 require 'onebox/json_ld'
 
-describe Onebox::JsonLd do
+RSpec.describe Onebox::JsonLd do
   it 'logs warning and returns an empty hash if received json is invalid' do
     invalid_json = "{\"@type\":invalid-json}"
     doc = Nokogiri::HTML("<script type=\"application/ld+json\">#{invalid_json}</script>")