Set up CI for 2.5-stable branch (#800)

Merge pull request 800

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: Continuous Integration
+on:
+  push:
+    branches:
+      - "2.5-stable"
+  pull_request:
+    branches:
+      - "2.5-stable"
+jobs:
+  build:
+    name: "Test with Jekyll ${{ matrix.jekyll }} (Ruby ${{ matrix.ruby }})"
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        jekyll: ["~> 3.9", "~> 4.2"]
+        ruby: ["2.7", "3.3"]
+    env:
+      JEKYLL_VERSION: ${{ matrix.jekyll }}
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+      - name: "Set Up Ruby ${{ matrix.ruby }}"
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+      - name: Run tests
+        run: script/cibuild

Gemfile

@@ -4,3 +4,4 @@ source "https://rubygems.org"
 gemspec
 
 gem "jekyll", ENV["JEKYLL_VERSION"] if ENV["JEKYLL_VERSION"]
+gem "kramdown-parser-gfm" if ENV["JEKYLL_VERSION"] == "~> 3.9"