Minor tidying of DivinerWorkflow classes

Summary: Minor tidying and modernizing a few things.

Test Plan: Ran ./bin/diviner atomize and ./bin/diviner generate.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11587