[wip] Add bundler dependency from development_dependency

In https://github.com/sanemat/tachikoma/blob/d0770f85d3f18f6ad2f081f7a3a7ea2712b22c43/lib/tasks/app.rake , tachikoma uses `Bundler.with_clean_env`.

This is enough? I need to add `require 'bundler'`?