Set minimum CI version to 2.4

Author: jeremyevans

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
   ruby-versions:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
-      min_version: 2.5
+      min_version: 2.4
   test:
     needs: ruby-versions
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})