Wrap ruby versions in strings

Author: ahayworth

.github/workflows/main.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: [2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6]
+        ruby-version: ["2.0", "2.1", "2.2", "2.3", "2.4", "2.5", "2.6"]
     steps:
     - name: Checkout code
       uses: actions/checkout@v1