Try to use released version of test-unit-ruby-core and Ruby 2.5

Author: hsbt

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby
-      min_version: 2.6
+      min_version: 2.5
 
   build:
     needs: ruby-versions
@@ -24,6 +24,5 @@ jobs:
       with:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
-        rubygems: latest
     - name: Run test
       run: bundle exec rake test

Gemfile

@@ -4,4 +4,4 @@ gemspec
 
 gem "rake"
 gem "test-unit"
-gem "test-unit-ruby-core", git: "https://github.com/ruby/test-unit-ruby-core"
+gem "test-unit-ruby-core"