Avoid using ruby 2.6

Author: taylorific

.github/workflows/polymath-ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [2.4, 2.5, 2.6]
+        ruby: [2.4, 2.5, 3.0]
     runs-on: ubuntu-latest
     steps:
     - name: Checkout repository