First version of GitHub Actions based CI

.github/workflows/ci.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: CI
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  rspec:
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        ruby:
+          - 2.7
+          - 2.6
+          - 2.5
+          - 2.4
+          - 2.3
+          - 2.2
+          - 2.1
+          - head
+
+          - jruby-9.1
+          - jruby-9.2
+          - jruby-head
+
+          - truffleruby
+          - truffleruby-head
+        include:
+          - ruby: 2.1
+            bundler-version: 1
+          - ruby: 2.2
+            bundler-version: 1
+
+    continue-on-error: ${{ endsWith(matrix.ruby, 'head') || matrix.ruby == 'truffleruby' }}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-version: ${{ matrix.bundler-version || '2' }}
+          bundler-cache: true
+
+      - name: rspec
+        run: bundle exec rspec