ci: update ruby versions to test

Remove 2.3 and include 2.7 at versions which the macOS builds do support.

Author: carlosmn

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.3.7', '2.4.6', '2.5.5', '2.6.3' ]
+        ruby: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0' ]
         os: [ ubuntu-18.04, macOS-10.14 ]
 
     runs-on: ${{ matrix.os }}