Enable Coverall's coverage report

Author: satoryu

.github/workflows/ruby.yml

@@ -28,4 +28,6 @@ jobs:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
     - name: Run tests
+      env:
+        COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}
       run: bundle exec rake spec