Allow to define cache in `.gitlab-ci.yml`

This extends .gitlab-ci.yml syntax to allow specifying caching files and directories between builds, making it easy to preserve ex. gems.

cache:
  paths:
  - .bundle
  - vendor/

before_script:
- bundle install --path vendor/

rspec:
  script:
  - bundle exec rspec

This is based on Build Artifacts changes.

/cc@dzaporozhets