Mirrored dependencies to increase reproducibility, security, and speed

After we support artifacts we can look into mirroring dependencies automatically.

As a user when I use GitLab CI all dependencies should be mirrored in GitLab automatically.

See https://www.jfrog.com/blog/fully-reproducible-builds-with-circleci-and-artifactory/ for an example.

Mirroring will increase reproducibility, security, and speed.

1. Reproducibility: if someone yanks a package or if the central repository (rubygems.org) is down the build still works.
2. Security: we can detect changes to a package.
3. Speed: getting the dependency from GitLab when it is needed again will be faster than getting it from a central place.