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
See merge request !1786
9573cf6cc7