Upgrade Ruby to version 2.6.8

So get the latest security fixes in the 2.6.x series and we can update
the parser gem without getting any warnings.

.gitlab-ci.yml

@@ -3,7 +3,7 @@ stages:
   - lint
 
 tests:
-  image: "ruby:2.6.7"
+  image: "ruby:2.6.8"
   stage: test
   services:
     - postgres:10.10
@@ -34,7 +34,7 @@ tests:
 # PRONTO_GITLAB_API_PRIVATE_TOKEN to your repository CI/CD settings
 # giving it the value of the Personal Access Token
 linters:
-  image: "ruby:2.6.7"
+  image: "ruby:2.6.8"
   stage: lint
   cache:
     key: consul