Upgrade cache key to be clear that Debian Stretch is being used

This fixes build errors caused by a stale cache since the new Docker image
upgraded from Debian Jessie to Stretch, which shipped an updated version
of OpenSSL.

.gitlab-ci.yml

@@ -6,7 +6,7 @@ image: "dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.3.7-golang-1.9-git
     - gitlab-org
 
 .default-cache: &default-cache
-  key: "ruby-2.3.7-with-yarn"
+  key: "ruby-2.3.7-debian-stretch-with-yarn"
   paths:
     - vendor/ruby
     - .yarn-cache/
@@ -550,7 +550,7 @@ static-analysis:
   script:
     - scripts/static-analysis
   cache:
-    key: "ruby-2.3.7-with-yarn-and-rubocop"
+    key: "ruby-2.3.7-debian-stretch-with-yarn-and-rubocop"
     paths:
       - vendor/ruby
       - .yarn-cache/