Add ruby 3.1 to test suite (#1553)

Author: enomotodev

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby-version: [2.5, 2.6, 2.7, 3.0]
+        ruby-version: [2.5, 2.6, 2.7, '3.0', 3.1]
 
     steps:
     - uses: actions/checkout@v2