Also test on last 2 macos and last 2 ubuntu

.github/workflows/ci.yml

@@ -4,15 +4,13 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental == true }}
     name: ruby-${{ matrix.ruby-version }}
     strategy:
       matrix:
+        os: [ubuntu-18.04, ubuntu-20.04, macos-10.15, macos-11.0]
         ruby-version: [3.0, 2.7, 2.6, 2.5]
-        include:
-          - ruby-version: head
-            experimental: true
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby