Add GitHub Actions CI

.ci.gemfile

@@ -0,0 +1,9 @@
+source 'https://rubygems.org'
+gemspec
+
+gem 'rake', '<10.0.0'
+
+if RUBY_VERSION < '2.4.0'
+  # Until mintest 5.12.0 is fixed
+  gem 'minitest', '5.11.3'
+end

.github/workflows/ci.yml

@@ -0,0 +1,25 @@
+name: CI
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [ 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, "3.0", jruby-9.1.17.0, jruby-9.2.13.0 ]
+    name: ${{ matrix.ruby }}
+    env:
+      BUNDLE_GEMFILE: .ci.gemfile
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - run: bundle exec rake