ci: use latest rubygems

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/linux.yml

@@ -23,6 +23,7 @@ jobs:
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        rubygems: latest
     - name: unit testing
       env:
         CI: true