Add MacOS to CI

.github/workflows/ci.yml

@@ -2,15 +2,17 @@ name: test
 on: [push, pull_request]
 jobs:
   build:
-    runs-on: ubuntu-latest
+    name: ${{ matrix.os }} Ruby ${{ matrix.ruby }}
+    runs-on: ${{ matrix.os }}-latest
     strategy:
       matrix:
+        os:   ['ubuntu', 'macos']
         ruby: [ '2.5', '2.6', '2.7' ]
-    name: Ruby ${{ matrix.ruby }}
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
+      - run: gem install bundler
       - run: bundle install
       - run: bundle exec rake test