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* Remove outdated option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

plugins/poll/spec/integration/poll_endpoints_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-describe "DiscoursePoll endpoints" do
+RSpec.describe "DiscoursePoll endpoints" do
   describe "fetch voters for a poll" do
     fab!(:user) { Fabricate(:user) }
     fab!(:post) { Fabricate(:post, raw: "[poll public=true]\n- A\n- B\n[/poll]") }