Replace Travis CI with GitHub Actions for running tests

- Remove Travis CI configuration
- Add GitHub Actions workflow to test with all compatible Ruby versions

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: Tests
+on: [push]
+
+jobs:
+  ruby:
+    name: Ruby Tests
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby-version: [ "2.3", "2.4", "2.5", "2.6", "2.7", "3.0" ]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+          ruby-version: ${{ matrix.ruby-version }}
+      - name: Run Tests
+        run: bundle exec rake test