Merge branch 'ruby-2-3-1' into 'master'

Update ruby 2.3.1

We where using 2.3.0, now 2.3.1

cc @connorshea 

See merge request !5790

.gitlab-ci.yml

-image: "ruby:2.3"
+image: "ruby:2.3.1"
 
 cache:
-  key: "ruby-23"
+  key: "ruby-231"
   paths:
   - vendor/apt
   - vendor/ruby