Add GitHub Actions CI

.github/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+jobs:
+  tests:
+    strategy:
+      matrix:
+        ruby-version: ['2.1', '2.2', '2.3', '2.4', '2.5', '2.6', '2.7']
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby-version }}
+          bundler-cache: true
+
+      - name: Run tests
+        run: bundle exec rake