Use Ruby v2.6 vs v2.2 to run lint CI job

It seems as if recent versions of minitest now require a more recent
version of Ruby. Without this change we were seeing the following error
during the `bundle install` step:

    minitest-5.18.0 requires ruby version < 4.0, >= 2.6, which is incompatible with

.circleci/config.yml

@@ -53,7 +53,7 @@ jobs:
             - run: MOCHA_GENERATE_DOCS=1 GOOGLE_ANALYTICS_WEB_PROPERTY_ID=dummy rake yardoc
   lint:
     docker:
-      - image: ruby:2.2
+      - image: ruby:2.6
     steps:
       - checkout
       - run: ruby --version