Add a github action for CI

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+on: [push, pull_request]
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest]
+        # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
+        ruby: [2.3, 2.4, 2.5, 2.6, 2.7, '3.0']
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - run: bundle exec rake