Reduce the test matrix as it's getting too large

* It was 40 checks and a lot of queuing on macOS.
* Version parsing is stable and can be tested more directly.
* 2.6.5 was tested twice: .ruby-version and explicitly.

Author: eregon

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         os: [ ubuntu-16.04, ubuntu-18.04, macos-latest, windows-latest ]
         # Use various version syntaxes here for testing
-        ruby: [ .ruby-version, 2.3, ruby-2.4, 2.5, ruby-2.6.5, 2.7.0, ruby-head, jruby, truffleruby, truffleruby-head, rubinius ]
+        ruby: [ .ruby-version, 2.3, 2.7.0, ruby-head, jruby, truffleruby, truffleruby-head, rubinius ]
         exclude:
         - os: windows-latest
           ruby: truffleruby