@starsirius: Run librarian:config and librarian:init as part of momentum init

This makes the `momentum:init` rake task a little more useful. Now it creates the correct .librarian/chef/config file and runs `librarian-chef init` to create an empty Cheffile.

Also, I discovered the `Bundler.with_clean_env` method, which removes all of the ENV/PATH-wrapping that Bundler normally does and lets you invoke non-bundled gems. Useful!