Update GitHub action matrix to test under 4 Ruby versions

Author: hoppergee

.github/workflows/main.yml

@@ -14,7 +14,10 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - 3.0.1
+          - 2.6
+          - 2.7
+          - 3.0
+          - 3.1
 
     steps:
     - uses: actions/checkout@v2