Migrate from TravisCI to GitHub Actions

Replaced outdated TravisCI configuration with modern GitHub Actions workflow.

Changes:
- Added .github/workflows/ci.yml with GitHub Actions configuration
- Removed .travis.yml (TravisCI configuration)
- Updated to use Ruby 3.4.1 (was 2.6.5)
- Replaced PhantomJS setup with modern Selenium WebDriver
- Added bundler caching for faster builds
- Added security audit step with bundle-audit
- Runs tests on push to main and on pull requests

The new workflow:
- Uses latest GitHub Actions (checkout@v4, ruby/setup-ruby@v1)
- Automatically caches gems for faster subsequent runs
- Sets up test database properly before running tests
- Runs RSpec tests in maintainer mode
- Includes optional security audit check

Fixes #491

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/ci.yml

@@ -0,0 +1,41 @@
+name: CI
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        ruby-version: ['3.4.1']
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby-version }}
+        bundler-cache: true
+
+    - name: Install dependencies
+      run: bundle install
+
+    - name: Setup database
+      run: |
+        RAILS_ENV=test bin/rails db:create
+        RAILS_ENV=test bin/rails db:migrate
+
+    - name: Run tests
+      env:
+        RAILSGOAT_MAINTAINER: true
+      run: bundle exec rspec
+
+    - name: Run security audit
+      run: bundle exec bundle-audit check --update
+      continue-on-error: true