Merge branch 'ruby-23-cache' into 'master'

Add caching for Ruby 2.3 tests.

Accidentally removed when !3807 was merged.

See merge request !4868

.gitlab-ci.yml

@@ -134,6 +134,11 @@ spinach 9 10: *spinach-knapsack
   image: "ruby:2.3"
   only:
     - master
+  cache:
+    key: "ruby-23"
+    paths:
+      - vendor/apt
+      - vendor/ruby
 
 .rspec-knapsack-ruby23: &rspec-knapsack-ruby23
   <<: *rspec-knapsack