CI: Disable global caching and use default caching

from https://docs.gitlab.com/ce/ci/yaml/README.html#cache:

"By default caching is enabled and shared between pipelines and jobs,
starting from GitLab 9.0"

.gitlab-ci.yml

 ---
 image: 0xacab.org:4567/leap/docker/ruby:latest
 
-# This is for caching the gems not only between the stages, but also
-# persistent on the gitlab-runner so we don't need to install from
-# scratch on every pipeline
 cache:
-  key: "global_platform_cache_between_pipelines"
   untracked: true
   paths:
+    # Cache gems
     - tests/platform-ci/vendor/
 
 before_script: