Add dependency on librarian (provided that librarian-chef is extracted)

Still waiting to hear whether Jay is amenable to extracting librarian-chef into its own project and removing the chef dependency in librarian itself, but wanted to get a headstart so that it's simply a matter of merging PR's when/if the time comes :)

Discussion PR: https://github.com/applicationsonline/librarian/pull/136
Chef-less librarian: https://github.com/patcon/librarian