Run tests using GitHub workflows (#2425)

travis-ci.org now displays "Please be aware travis-ci.org will be
shutting down in several weeks, with all accounts migrating to
travis-ci.com. Please stay tuned here for more information."

As GitHub actions is free for public repositories, whereas it's unclear
to what travis-ci.com will be, and the switching cost is low, it seems
going with workflows is the prudent thing to do.

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: CI
+on:
+  push:
+    branches: [ $default-branch ]
+  pull_request:
+    branches: [ $default-branch ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    continue-on-error: ${{ matrix.experimental == true }}
+    name: ruby-${{ matrix.ruby-version }}
+    strategy:
+      matrix:
+        ruby-version: [head, 3.0, 2.7, 2.6, 2.5]
+        include:
+          - ruby-version: head
+            experimental: true
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby-version }}
+          bundler-cache: true
+      - name: Runs tests
+        run: bundle exec rake
+        env:
+          test: true

.travis.yml

@@ -1,18 +0,0 @@
-language: ruby
-dist: xenial
-cache: bundler
-rvm:
-  - ruby-head
-  - 3.0
-  - 2.7
-  - 2.6
-  - 2.5
-os:
-  - linux
-jobs:
-  fast_finish: true
-  allow_failures:
-    - rvm: ruby-head
-env:
-  global:
-    - TEST=true