Try to use setup-ruby on macos

Author: hsbt

.github/workflows/macos.yml

@@ -5,8 +5,15 @@ on: [push]
 jobs:
   build:
     runs-on: macos-latest
+    strategy:
+      matrix:
+        ruby: [ '2.6.x', '2.5.x', '2.4.x', '2.3.x' ]
     steps:
     - uses: actions/checkout@master
+    - name: Set up Ruby
+      uses: actions/setup-ruby@v1
+      with:
+        version: ${{ matrix.ruby }}
     - name: Install dependencies
       run: gem install minitest
     - name: Run test