Add some GH-based testing

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+- package-ecosystem: bundler
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "13:00"
+  open-pull-requests-limit: 10

.github/workflows/test.yml

@@ -0,0 +1,26 @@
+name: Test
+
+on:
+  - pull_request
+  - push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby:
+          - "2.5"
+          - "2.6"
+          - "2.7"
+    name: Ruby ${{ matrix.ruby }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Ruby ${{ matrix.ruby }}
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+      - name: Run tests
+        run: bundle exec rake test:spec