CI: Use latest Bundler to support Ruby head

Ruby 4.1.0dev (head) removed Pathname::SEPARATOR_PAT, which caused
Bundler 2.4.22 (specified in Gemfile.lock) to fail with a NameError.

By setting 'bundler: latest', setup-ruby will install a version of
Bundler compatible with the running Ruby version, resolving the
incompatibility in the head build while maintaining Ruby 2.7 support.

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

.github/workflows/test.yml

@@ -30,7 +30,7 @@ jobs:
         uses: ruby/setup-ruby@c4e5b1316158f92e3d49443a9d58b31d25ac0f8f # v1.306.0
         with:
           ruby-version: ${{ matrix.ruby }}
-          bundler-cache: true
+          bundler: latest
       - name: Run tests
         run: bundle exec rake test
       - name: Upload coverage results

Gemfile.lock

@@ -180,6 +180,3 @@ DEPENDENCIES
   vcr
   webmock
   yajl-ruby
-
-BUNDLED WITH
-   2.4.22