ci: use Ruby 2.7 & update actions (#35)

For now I'm just bumping to v2.7 to unblock CI

.github/workflows/ruby.yml

@@ -2,21 +2,21 @@ name: Ruby
 
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 
 jobs:
   build:
 
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
-    - name: Set up Ruby 2.6
-      uses: actions/setup-ruby@v1
+    - uses: actions/checkout@v3
+    - name: Set up Ruby 2.7
+      uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.6.x
+        ruby-version: '2.7'
     - name: Build and test with Rake
       run: |
         gem install bundler