Test on Ruby 3.1

Author: lovro-bikic

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [2.6, 2.7, 3.0]
+        ruby: [2.6, 2.7, '3.0', 3.1]
 
     steps:
       - uses: actions/checkout@v2