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* Remove outdated option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/discourse_hub_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe DiscourseHub do
+RSpec.describe DiscourseHub do
   describe '.discourse_version_check' do
     it 'should return just return the json that the hub returns' do
       hub_response = { 'success' => 'OK', 'latest_version' => '0.8.1', 'critical_updates' => false }