Test all Ruby minor versions

* Skip tests on the older ubuntu-16.04 to have a reasonable number of jobs

Author: eregon

.github/workflows/test.yml

@@ -14,9 +14,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04, macos-latest, windows-latest ]
+        os: [ ubuntu-latest, macos-latest, windows-latest ]
         # Use various version syntaxes here for testing
-        ruby: [ 2.2, 2.3, .ruby-version, .tool-versions, ruby-head, jruby-9.1, jruby, jruby-head, truffleruby, truffleruby-head ]
+        ruby: [ 2.2, 2.3, 2.4, 2.5, 2.6.5, 2.7, ruby-head, jruby-9.1, jruby, jruby-head, truffleruby, truffleruby-head ]
         exclude:
         - os: windows-latest
           ruby: truffleruby