Use RSpec 4 pre-release

RSpec 4 will eventually be released. Since we're checking its style, why
don't we add a job to use it to run our specs.
This will help a bit to test RSpec 4 out.

.github/workflows/main.yml

@@ -61,3 +61,24 @@ jobs:
           ruby-version: "${{ matrix.ruby }}"
           bundler-cache: true
       - run: bundle exec rake ${{ matrix.task }}
+
+  rspec4:
+    runs-on: ubuntu-latest
+    name: RSpec 4
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use latest RSpec 4 from `4-0-dev` branch
+        run: |
+          sed -e '/rspec/d' -i Gemfile
+          cat << EOF > Gemfile.local
+            gem 'rspec', github: 'rspec/rspec', branch: '4-0-dev'
+            gem 'rspec-core', github: 'rspec/rspec-core', branch: '4-0-dev'
+            gem 'rspec-expectations', github: 'rspec/rspec-expectations', branch: '4-0-dev'
+            gem 'rspec-mocks', github: 'rspec/rspec-mocks', branch: '4-0-dev'
+            gem 'rspec-support', github: 'rspec/rspec-support', branch: '4-0-dev'
+          EOF
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+          bundler-cache: true
+      - run: bundle exec rake spec

.rspec

@@ -1,3 +1,2 @@
 --require spec_helper
---color
 --format documentation

Gemfile

@@ -6,7 +6,7 @@ gemspec
 
 gem 'rack'
 gem 'rake'
-gem 'rspec', '>= 3.4'
+gem 'rspec', '~> 3.11'
 gem 'rubocop-performance', '~> 1.7'
 gem 'rubocop-rake', '~> 0.6'
 gem 'yard'

spec/smoke_tests/weird_rspec_spec.rb

@@ -86,7 +86,7 @@
   end
 
   it_behaves_like :something
-  it_should_behave_like :something
+  it_should_behave_like :something if RSpec::Core::Version::STRING < '4.0'
 
   it_behaves_like :something do
     let(:foo) { 'bar' }

spec/spec_helper.rb

@@ -24,11 +24,7 @@ module SpecHelper
   config.order = :random
 
   # Run focused tests with `fdescribe`, `fit`, `:focus` etc.
-  config.filter_run focus: true
-  config.run_all_when_everything_filtered = true
-
-  # Forbid RSpec from monkey patching any of our objects
-  config.disable_monkey_patching!
+  config.filter_run_when_matching :focus
 
   # We should address configuration warnings when we upgrade
   config.raise_errors_for_deprecations!