CI changes

run CI for main and PR, not both, and cache bundle install, add some
newer ruby versions

.github/workflows/main.yml

@@ -1,18 +1,22 @@
 name: Ruby
 
-on: [push,pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   build:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ 2.7, '3.0', 3.1, ruby-head ]
+        ruby: [ 2.7, 3.1, 3.2, 3.3, ruby-head ]
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - run: bundle install
+        bundler-cache: true
     - run: bundle exec rake