circle.yml in mutant-0.8.14 vs circle.yml in mutant-0.8.15

@@ -8,10 +8,10 @@
 version: 2
 jobs:
   ruby_2_3:
     <<: *defaults
     docker:
-      - image: circleci/ruby:2.3.3
+      - image: circleci/ruby:2.3.4
   ruby_2_4:
     <<: *defaults
     docker:
       - image: circleci/ruby:2.4.1
 workflows: