Test different versions of Ubuntu

Author: etagwerker

.github/workflows/ci.yml

@@ -12,6 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
+        os: [ubuntu-18.04, ubuntu-20.04]
         ruby: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2']
     steps:
     - uses: actions/checkout@v2