ci: add previous OS version dimensions

Author: raggi

.github/workflows/ruby.yml

@@ -15,18 +15,19 @@ on:
 
 jobs:
   test:
-
     runs-on: ${{ matrix.os }}
     strategy:
+      fail-fast: false
+
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-latest, ubuntu-18.04, macos-10.15]
         ruby-version: ['2.6', '2.7', '3.0']
 
     steps:
     - uses: actions/checkout@v2
     - name: Brew automake
       run: brew install automake
-      if: matrix.os == 'macos-latest'
+      if: startsWith(matrix.os, 'macOS')
     - name: Set up Ruby
     # To automatically get bug fixes and new Ruby versions for ruby/setup-ruby,
     # change this to (see https://github.com/ruby/setup-ruby#versioning):