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* Remove outdated option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/theme_store/git_importer_spec.rb

@@ -4,7 +4,7 @@
 
 require 'theme_store/git_importer'
 
-describe ThemeStore::GitImporter do
+RSpec.describe ThemeStore::GitImporter do
 
   describe "#import" do
 

spec/lib/theme_store/zip_exporter_spec.rb

@@ -2,7 +2,7 @@
 
 require 'theme_store/zip_exporter'
 
-describe ThemeStore::ZipExporter do
+RSpec.describe ThemeStore::ZipExporter do
   let(:rand_hex) do
     +"X" << SecureRandom.hex
   end

spec/lib/theme_store/zip_importer_spec.rb

@@ -4,7 +4,7 @@
 
 require 'theme_store/zip_importer'
 
-describe ThemeStore::ZipImporter do
+RSpec.describe ThemeStore::ZipImporter do
   before do
     @temp_folder = "#{Pathname.new(Dir.tmpdir).realpath}/discourse_theme_#{SecureRandom.hex}"