Migrate from Travis CI to GitHub Actions

- Add GitHub Actions workflow testing Ruby 2.7, 3.0, 3.1, 3.2, 3.3
- Remove outdated Travis CI config (was testing Ruby 2.3-2.5)
- Update README badge

Ruby 3.3 compatibility fixes:
- Update rubocop from 0.71.0 to ~> 1.68 (0.71.0 incompatible with Ruby 3.3)
- Simplify .rubocop.yml - Rails and Performance cops were extracted to
  separate gems (rubocop-rails, rubocop-performance) in RuboCop 0.72.
  Since sift doesn't need those cops, removed them instead of adding
  the extra dependencies.
- Add `require "ostruct"` in test_helper (ostruct moved to gem in Ruby 3.2+)
- Pin minitest to ~> 5.0 for Rails 7.x appraisals (minitest 6.0 has
  breaking API changes incompatible with Rails test runner)
- Pin sqlite3 to ~> 1.4 for Rails 7.0/7.1 (Rails 7.0 requires sqlite3 < 2)

Rails version updates:
- Drop Rails 6.1 (end of life, no longer receiving security fixes)
- Add Rails 7.1, 7.2, 8.0 appraisals
- Update activerecord dependency to >= 7.0

Author: rafafloresta

.github/workflows/test.yml

@@ -0,0 +1,42 @@
+name: Tests
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3']
+        gemfile:
+          - rails_70
+          - rails_71
+          - rails_72
+          - rails_80
+        exclude:
+          # Rails 7.2 requires Ruby >= 3.1
+          - ruby: '2.7'
+            gemfile: rails_72
+          - ruby: '3.0'
+            gemfile: rails_72
+          # Rails 8.0 requires Ruby >= 3.2
+          - ruby: '2.7'
+            gemfile: rails_80
+          - ruby: '3.0'
+            gemfile: rails_80
+          - ruby: '3.1'
+            gemfile: rails_80
+    env:
+      BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+      - run: bundle exec rake test

.gitignore

@@ -2,6 +2,7 @@
 log/*.log
 *.swp
 Gemfile.lock
+gemfiles/*.lock
 pkg/
 test/dummy/db/*.sqlite3
 test/dummy/db/*.sqlite3-journal