Stop using Travis CI and switch to GitHub Actions

We previously used Travis CI conveniently, but as of 2024, [using Travis CI to build OSS requires contacting them to get a fixed amount of credits each month](https://docs.travis-ci.com/user/billing-faq/#what-if-i-am-building-open-source), making it cumbersome. We want to reduce the hassle by migrating from Travis CI to GitHub Actions.

In the migration to GitHub Actions, we have configured CI to work with Ruby up to version 3.3. Additionally, since [@ruby/setup-ruby](https://github.com/ruby/setup-ruby) does not support Rubinius, it has been excluded. Older versions of JRuby are not supported either, so we have only specified jruby and jruby-head.

Author: willnet

.github/workflows/ci.yml

@@ -0,0 +1,23 @@
+name: Ruby CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby-version: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', 'head', 'jruby', 'jruby-head']
+    continue-on-error: ${{ matrix.ruby-version == 'jruby-head' || matrix.ruby-version == 'head' }}
+    env:
+      JRUBY_OPTS: '--debug'
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby-version }}
+          bundler-cache: true
+      - name: Run tests
+        run: bundle exec rake