feat: add github action for CI (rspec)

Using a matrix to test on different ruby versions.

.github/workflows/main.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  tests:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-18.04]
+        # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
+        ruby: [2.5, 2.6, 2.7, '3.0']
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup Ruby and install gems
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+
+      - name: Ruby tests
+        run: bundle exec rspec