Replace Travis CI with GitHub Actions

Travis CI stopped free OSS builds in 2021. Add a GitHub Actions
workflow that tests Ruby 3.3, 3.4, and 4.0 on every push to main
and on all pull requests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: agrberg

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['3.3', '3.4', '4.0']
+
+    name: Ruby ${{ matrix.ruby }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+
+      - run: bundle exec rake