Replace Travis CI build with GitHub Action

travis-ci.org is shutting down and Open Source support will be severely
limited on travis-ci.com. Build with GitHub Actions instead.

Differences with the existing build:
- Added 3.0
- Removed ruby-head. GitHub Actions always fail when one of the builds
  fails.

Author: mvz

.github/workflows/ruby.yml

@@ -0,0 +1,31 @@
+# This workflow will download a prebuilt Ruby version, install dependencies and
+# run tests with Rake
+# For more information see: https://github.com/marketplace/actions/setup-ruby-jruby-and-truffleruby
+
+name: Ruby
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  test:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        ruby: [2.5, 2.6, 2.7, "3.0"]
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - name: Run tests
+      run: bundle exec rake spec