Merge pull request #99 from fluent/ci-update-ruby

CI: Update Ruby

Author: ashie

.github/workflows/linux.yml

@@ -1,14 +1,16 @@
-name: Testing on Ubuntu
+name: Test on Ubuntu
 on:
-  - push
-  - pull_request
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
 jobs:
   build:
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.4', '2.5', '2.6', '2.7' ]
+        ruby: [ '3.1', '3.0', '2.7', '2.6' ]
         os:
           - ubuntu-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}