Selectively redirect submodules to the cached repo

This mimics what was done in https://github.com/square/kochiku/pull/26 for Kochiku. Unfortunately the `git_repo.rb` file have diverged between the two projects so the code change is not identical.

Every time I change `git_repo.rb` it gives me motivation to merge the two projects into a single repo... I'd love to do that sometime.

@square/kochiku-contributors