Try adding CI

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: Ruby CI
+
+on:
+  push:
+    branches: [$default-branch]
+  pull_request:
+    branches: [$default-branch]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby-version: [2.7.x, 2.6.x, 2.5.x, 2.4.x]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Ruby ${{ matrix.ruby-version }}
+        # To automatically get bug fixes and new Ruby versions for ruby/setup-ruby,
+        # change this to (see https://github.com/ruby/setup-ruby#versioning):
+        # uses: ruby/setup-ruby@v1
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby-version }}
+          bundler-cache: true
+      - name: Install dependencies
+        run: bundle install
+      - name: Run tests
+        run: bundle exec rake