Update CI build with new setup-ruby (#53)

Replace the old GitHub Actions setup-ruby with its replacement

.github/workflows/jekyll.yaml

@@ -13,7 +13,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby
-      uses: actions/setup-ruby@v1
+      uses: ruby/setup-ruby@v1
       with:
         ruby-version: 2.6
     - name: Cache gems