Merge pull request #85 from fluent/ci-update-ruby

CI: Update Ruby

Author: ashie

.github/workflows/linux.yml

@@ -1,15 +1,17 @@
 name: Testing on Ubuntu
 on:
-  - push
-  - pull_request
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
 jobs:
   build:
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
+        ruby: [ '3.1', '3.0', '2.7', '2.6' ]
         os:
           - ubuntu-latest
         experimental: [false]