Update Jekyll to v4.0.0.

.github/workflows/test.yml

@@ -52,6 +52,9 @@ jobs:
       - name: Set up Bundler
         run: gem install bundler -v "~> 1.17"
 
+      - name: Update rubygems
+        run: gem update --system
+
       - run: ruby --version
       - run: gem --version
       - run: bundle --version