Add CI GitHub workflow

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    name: Ruby ${{ matrix.ruby }}
+    strategy:
+      matrix:
+        ruby: [2.6, 2.7, 3.0, 3.1]
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v2
+    - name: Set up Ruby ${{ matrix.ruby }}
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - name: Ruby tests
+      run: bundle exec rspec