Bundle support

Fixes #13580, #13584, #13587

TODO:
- [x] Add `jekyll-sitemap` plugin
- [x] Switch to kramdown
- [ ] Update Readme with `bundle` info
- [ ] Make Travis work again
  - [ ] Move `Gemfile.lock` into `/test-infra/`
  - [ ] Switch from `$GEMDIR` to wherever Bundler stores gems