Switch from Travis CI to GitHub CI

It seems Travis CI is no longer available for pull requests and forks
to run tests. In order for this to work by default and give feedback
early, try GitHub CI instead.

While at it, remove Ruby 2.3 and 2.4 from the test matrix because
those were failing with:

```
 kramdown was resolved to 2.5.1, which depends on
      Ruby (>= 2.5)

  Current Ruby version:
    Ruby (= 2.4.10)
```

Author: Krinkle

.github/workflows/CI.yaml

@@ -0,0 +1,63 @@
+name: CI
+on:
+  push:
+
+jobs:
+  lint:
+    name: Rubocop
+    runs-on: ubuntu-latest
+    steps:
+
+    - uses: actions/checkout@v4
+
+    - name: Install Rubocop
+      run: |
+        sudo gem install rubocop -v '~> 1.70'
+        sudo gem install rubocop-performance -v '~> 1.23'
+
+    - name: Run Rubocop
+      run: rubocop --fail-level error
+
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - ruby: '2.5'
+          - ruby: '2.6'
+          - ruby: '2.7'
+          - ruby: '3.0'
+          - ruby: '3.1'
+          - ruby: '3.2'
+          - ruby: jruby-head
+            experimental: true
+
+    name: Ruby ${{ matrix.ruby }}
+    continue-on-error: ${{ matrix.experimental == true }}
+    runs-on: ubuntu-24.04
+    steps:
+
+    - uses: actions/checkout@v4
+
+    # https://github.com/ruby/setup-ruby
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+
+    - name: Install packages
+      run: |
+        ruby --version
+        gem --version
+        gem install rake
+        wget https://github.com/htacg/tidy-html5/releases/download/5.4.0/tidy-5.4.0-64bit.deb
+        sudo apt purge libtidy-0.99-0
+        sudo dpkg -i tidy-5.4.0-64bit.deb
+
+    - name: Install dependencies
+      run: |
+        rake gemspec
+        echo -e 'source "http://www.rubygems.org"\n\ngemspec' > Gemfile
+        bundle install
+
+    - name: Run tests
+      run: ruby -Ilib:test test/test_*