Update Bundler to latest for GHA

Author: ur5us

.github/workflows/main.yml

@@ -46,6 +46,7 @@ jobs:
           ruby-version: ${{ matrix.ruby_version }}
       - name: Build and run test
         run: |
+          gem update bundler
           bundle -v
           bundle
           bundle exec appraisal rails_${{ matrix.rails_version }} bundle