Update README.md for new Ruby version
Update README.md for new Ruby version

Small follow-up change from commit 48b647b1efd6cc2dff8f205af81c2effb121b514

empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core