Make default button text size 13px.

Summary: Makes buttons look 13px by default, 11 px small.

Test Plan: http://phab1.pushlabs.net/uiexample/view/PhabricatorButtonsExample/

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2078

Differential Revision: https://secure.phabricator.com/D3972