Replace Travis CI with GitHub Actions (#350)

Both on PR and push.

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: Ruby
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu
+        ruby:
+          - "2.5"
+          - "2.6"
+          - "2.7"
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - name: Install dependencies
+      run: bundle install
+    - name: Run tests
+      run: bundle exec rake