Add Test on ruby 2.4

Author: kojix2

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         os:   ['ubuntu', 'macos']
-        ruby: [ '2.6', '2.7', '3.0' ]
+        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0' ]
     steps:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1