Smaller Roboto Slab files

Summary: Generated Roboto Slab with the same settings as Lato, which produces a smaller file. Also hope it fixes mobile kerning.

Test Plan: View various documents, fonts look identical.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14388