Tweak representation of build matrix

Ruby 3.0 version specification in YAML should be string `'3.0'` instead of
float `3.0`. It will be `3` instead of the expected `3.0`.

Author: koic

.github/workflows/test.yml

@@ -11,10 +11,10 @@ jobs:
     strategy:
       matrix:
         ruby: [
-          3.0,
-          2.7,
-          2.6,
-          2.5,
+          '3.0',
+          '2.7',
+          '2.6',
+          '2.5',
           jruby,
           jruby-head
         ]